IFSM 310 – Cloud Computing vs. Client-Server Computing - DUE FRIDAY

Chapter 4

Internetworking Layer (IP)

Now that we can move data across a single link, it’s time to fig-

ure out how to move it across the country or around the world.

To send data from your computer to any of a billion destinations,

the data needs to move across multiple hops and across multiple

networks. When you travel from your home to a distant destina-

tion, you might walk from your home to a bus stop, take a train to

the city, take another train to the airport, take a plane to a differ-

ent airport, take a taxi into the city, then take a train to a smaller

town, a bus to an even smaller town, and finally walk from the bus

stop to your hotel. A packet also needs to take multiple forms of

transportation to reach its destination. For a packet taking its

“trip” to another country, the “walk”, “bus”, “train”, and “plane”

can be thought of as different link layers like WiFi, Ethernet, fiber

optic, and satellite.

At each point during the trip, you (or your packet) are being trans-

ported using a shared medium. There might be hundreds of other

people on the same bus, train, or plane, but your trip is different

from that of every other traveller because of the decisions that

you make at the end of each of your “hops”. For instance, when

you arrive at a train station, you might get off one train, then walk

through the station and select a particular outbound train to con-

tinue your journey. Travellers with different starting points and

destinations make a different series of choices. All of the choices

you make during your trip result in you following a series of links

(or hops) along a route that takes you from your starting point to

your destination.

As your packet travels from its starting point to its destination,

35

36 CHAPTER 4. INTERNETWORKING LAYER (IP)

Figure 4.1: Travelling Packets

it also passes through a number of “stations” where a decision

is made as to which output link your packet will be forwarded

on. For packets, we call these places “routers”. Like train sta-

tions, routers have many incoming and outgoing links. Some

links may be fiber optic, others might be satellite, and still others

might be wireless. The job of the router is to make sure packets

move through the router and end up on the correct outbound link

layer. A typical packet passes through from five to 20 routers as

it moves from its source to its destination.

But unlike a train station where you need to look at displays to

figure out the next train you need to take, the router looks at the

destination address to decide which outbound link your packet

needs to take. It is as if a train station employee met every single

person getting off an inbound train, asked them where they were

headed, and escorted them to their next train. If you were a

packet, you would never have to look at another screen with a

list of train departures and tracks!

The router is able to quickly determine the outbound link for your

packet because every single packet is marked with its ultimate

destination address. This is called the Internet Protocol Address,

or IP Address for short. We carefully construct IP addresses to

4.1. INTERNET PROTOCOL (IP) ADDRESSES 37

make the router’s job of forwarding packets as efficient as possi-

ble.

4.1 Internet Protocol (IP) Addresses

In the previous section where we talked about Link layer ad-

dresses, we said that link addresses were assigned when the

hardware was manufactured and stayed the same throughout

the life of a computer. We cannot use link layer addresses to

route packets across multiple networks because there is no rela-

tionship between a link layer address and the location where that

computer is connected to the network. With portable computers

and cell phones moving constantly, the system would need to

track each individual computer as it moved from one location to

another. And with billions of computers on the network, using

the link layer address to make routing decisions would be slow

and inefficient.

!"#$%&'"(

)&&*+,#(+'$

-$(."$.(/'"0

1+$0

Figure 4.2: The Internetwork Layer

To make this easier, we assign another address to every computer

based on where the computer is connected to the network. There

are two different versions of IP addresses. The old (classic) IPv4

addresses consist of four numbers separated by dots like this, and

look like this:

212.78.1.25

38 CHAPTER 4. INTERNETWORKING LAYER (IP)

Each of the numbers can only be from 0 through 255. We have

so many computers connected to the Internet now that we are

running out of IPv4 addresses to assign to them. IPv6 address are

longer and look like:

2001:0db8:85a3:0042:1000:8a2e:0370:7334

For this section we will focus on the classic IPv4 addresses, but all

of the ideas apply equally to IPv4 and IPv6 addresses.

The most important thing about of IP addresses is that they can

be broken into two parts.1 The first part of the two-piece address

is called the “Network Number”. If we break out an IPv4 address

into two parts, we might find the following:

Network Number: 212.78

Host Identifier: 1.25

The idea is that many computers can be connected via a single

connection to the Internet. An entire college campus, school, or

business could connect using a single network number, or only a

few network numbers. In the example above, 65,536 computers

could be connected to the network using the network number of

“212.78”. Since all of the computers appear to the rest of the

Internet on a single connection, all packets with an IP address of:

212.78.*.*

can be routed to the same location.

By using this approach of a network number and a host identifier,

routers no longer have to keep track of billions of individual com-

puters. Instead, they need to keep track of perhaps a million or

less different network numbers.

So when your packet arrives in a router and the router needs to

decide which outbound link to send your packet to, the router

does not have to look at the entire IP address. It only needs to

look at the first part of the address to determine the best out-

bound link.

1There are many points where an IP address can be broken into “Network

Number” and “Host Identifier” - for this example, we will just split the address

in half.

4.2. HOW ROUTERS DETERMINE THE ROUTES 39

4.2 How Routers Determine the Routes

While the idea of the collapsing many IP addresses into a single

network number greatly reduces the number of individual end-

points that a router must track to properly route packets, each

router still needs a way to learn the path from itself to each of the

network numbers it might encounter.

When a new core router is connected to the Internet, it does not

know all the routes. It may know a few preconfigured routes, but

to build a picture of how to route packets it must discover routes

as it encounters packets. When a router encounters a packet that

it does not already know how to route, it queries the routers that

are its “neighbors”. The neighboring routers that know how to

route the network number send their data back to the request-

ing router. Sometimes the neighboring routers need to ask their

neighbors and so on until the route is actually found and sent

back to the requesting router.

In the simplest case, a new core router can be connected to the

Internet and slowly build a map of network numbers to outbound

links so it can properly route packets based on the IP address for

each incoming packet. We call this mapping of network numbers

to outbound links the “routing table” for a particular router.

When the Internet is running normally, each router has a rela-

tively complete routing table and rarely encounters a new net-

work number. Once a router figures out the route to a new net-

work number the first time it sees a packet destined for that net-

work number, it does not need to rediscover the route for the

network number unless something changes or goes wrong. This

means that the router does a lookup on the first packet, but then

it could route the next billion packets to that network number just

by using the information it already has in its routing tables.

4.3 When Things Get Worse and Better

Sometimes the network has problems and a router must find a

way to route data around the problems. A common problem is

that one of the outbound links fails. Perhaps someone tripped

over a wire and unplugged a fiber optic cable. At this point, the

router has a bunch of network numbers that it wants to route

out on a link that has failed. The recovery when a router loses

an outbound link is surprisingly simple. The router discards all

40 CHAPTER 4. INTERNETWORKING LAYER (IP)

of the entries in its routing table that were being routed on that

link. Then as more packets arrive for those network numbers, the

router goes through the route discovery process again, this time

asking all the neighboring routers except the ones that can no

longer be contacted due to the broken link.

Figure 4.3: Dynamic Routing

Packets are routed more slowly for a while as routing tables are re-

built that reflect the new network configuration, but after a while

things are humming along nicely.

This is why it is important for there to always be at least two inde-

pendent paths from a source network to a destination network in

the core of the network. If there are always at least two possible

independent routes, we say that a network is a “two-connected

network”. A two-connected network can recover from any single

link outage. In places where there are a lot of network connec-

tions, like the east coast of the United States, the network could

lose many links without ever becoming completely disconnected.

But when you are at your home or school and have only one con-

nection, if that connection goes down you are disconnected com-

pletely.

At some point the broken link is repaired or a new link is brought

up, and the router wants to make best use of the new links. The

4.4. DETERMINING YOUR ROUTE 41

router is always interested in improving its routing tables, and

looks for opportunities to improve its routing tables in its spare

time. When there is a lull in communication, a router will ask a

neighboring router for all or part of its routing table. The router

looks through the neighbor’s tables and if it looks like the other

router has a better route to a particular network number, it up-

dates its network table to forward packets for that network num-

ber through the link to the router that has a better route.

With these approaches to outages and the exchange of routing

table information, routers can quickly react to network outages

and reroute packets from links that are down or slow to links that

are up and/or faster. All the while, each router is talking to its

neighboring routers to find ways to improve its own routing ta-

ble. Even though there is no central source of the “best route”

from any source to any destination, the routers are good at know-

ing the fastest path from a source to a destination nearly all the

time. Routers are also good at detecting and dynamically routing

packets around links that are slow or temporarily overloaded.

One of the side effects of the way routers discover the structure

of the network is that the route your packets take from the source

to the destination can change over time. You can even send one

packet immediately followed by another packet and because of

how the packets are routed, the second packet might arrive at

the destination before the first packet. We don’t ask the IP layer

to worry about the order of the packets; it already has enough to

worry about.

We pour our packets with source and destination IP addresses into

the Internet much like we would send out a bunch of letters in the

mail at the post office. The packets each find their way though

the system and arrive at their destinations.

4.4 Determining Your Route

There is no place in the Internet that knows in advance the route

your packets will take from your computer to a particular destina-

tion. Even the routers that participate in forwarding your packets

across the Internet do not know the entire route your packet will

take. They only know which link to send your packets to so they

will get closer to their final destination.

But it turns out that most computers have a network diagnostic

tool called “traceroute” (or “tracert”, depending on the operating

42 CHAPTER 4. INTERNETWORKING LAYER (IP)

system) that allows you to trace the route between your com-

puter and a destination computer. Given that the route between

any two computers can change from one packet to another, when

we “trace” a route, it is only a “pretty good guess” as to the actual

route packets will take.

The traceroute command does not actually “trace” your packet

at all. It takes advantage of a feature in the IP network protocol

that was designed to avoid packets becoming “trapped” in the

network and never reaching their destination. Before we take a

look at traceroute, let’s take a quick look at how a packet might

get trapped in the network forever and how the IP protocol solves

that problem.

Remember that the information in any single router is imperfect

and is only an approximation of the best outbound link for a par-

ticular network number, and each router has no way of knowing

what any other router will do. But what if we had three routers

with routing table entries that formed an endless loop?

!"!#$!#%#% !"!#$!#%#%

!"!#$!#%#% !"!#$!#%#%

!"!#$!#%#% !"!#$!#%#%

!"!#$!#%#% !"!#$!#%#%

Figure 4.4: Routing Vortex

Each of the routers thinks it knows the best outbound link for IP

addresses that start with “212.78”. But somehow the routers are

a little confused and their routing tables form a loop. If a packet

with a prefix of “212.78” found its way into one of these routers,

it would be routed around a circle of three links forever. There

is no way out. As more packets arrived with the same prefix,

they would just be added to the “infinite packet vortex”. Pretty

soon the links would be full of traffic going round and round, the

routers would fill up with packets waiting to be sent, and all three

routers would crash. This problem is worse than having someone

4.4. DETERMINING YOUR ROUTE 43

trip over a fiber optic cable, since it can cause several routers to

crash.

To solve this problem, the Internet Protocol designers added a

number to each packet that is called the Time To Live (TTL). This

number starts out with a value of about 30. Each time an IP

packet is forwarded down a link, the router subtracts 1 from the

TTL value. So if the packet takes 15 hops to cross the Internet, it

will emerge on the far end with a TTL of 15.

But now let’s look at how the TTL functions when there is a routing

loop (or “packet vortex”) for a particular network number. Since

the packet keeps getting forwarded around the loop, eventually

the TTL reaches zero. And when the TTL reaches zero, the router

assumes that something is wrong and throws the packet away.

This approach ensures that routing loops do not bring whole areas

of the network down.

So that is a pretty cool bit of network protocol engineering. To

detect and recover from routing loops, we just put a number in,

subtract 1 from that number on each link, and when the number

goes to zero throw the packet away.

It also turns out that when the router throws a packet away, it

usually sends back a courtesy notification, something like, “Sorry

I had to throw your packet away.” The message includes the IP

address of the router that threw the packet away.

Network loops are actually pretty rare, but we can use this notifi-

cation that a packet was dropped to map the approximate route a

packet takes through the network. The traceroute program sends

packets in a tricky manner to get the routers that your packets

pass through to send it back notifications. First, traceroute sends

a packet with a TTL of 1. That packet gets to the first router and

is discarded and your computer gets a notification from the first

router. Then traceroute sends a packet with a TTL of 2. That

packet makes it through the first router and is dropped by the

second router, which sends you back a note about the discarded

packet. Then traceroute sends a packet with a TTL of 3, and con-

tinues to increase the TTL until the packet makes it all the way to

its destination.

With this approach, traceroute builds up an approximate path that

your packets are taking across the network.

It took 14 hops to get from Ann Arbor, Michigan to Palo Alto, Cal-

ifornia. The packets passed through Kansas, Texas, Los Angeles,

and Oakland. This might not be the best route between the two

cities if you were driving a car or taking a train, but on that day

44 CHAPTER 4. INTERNETWORKING LAYER (IP)

!"#$%"&'!%()))*+!#,-&".*%.'

!"#$%"&'!%(!&()))*+,!#-.&"/+%/'(0121+32+45+6278(39(:&;,(<#=8(95(>?!%(;#$@%!,

(1((191+411+456+4*4(0191+411+456+4*47((1+6A5(<,((5+*69(<,((5+9A5(<,

(4((BC>D-C,%>+"C>D-C,%>+'<-%!+'<D$:+%/'(01A4+144+1E6+317((5+*A1(<,((5+**E(<,((5+*25(<,

(6((BC>D-C,%>CD4C##+<%"D!C##4+'<-%!+'<D$:+%/'(01A4+14+E5+667((3+315(<,((3+*9*(<,((3+3*9(<,

(9((1A4+144+1E6+65(01A4+144+1E6+657((2+A1A(<,((2+45A(<,((2+144(<,

(*((,&C9C6C5+5+"!"+@#-,+-%!+D-!%"-%!4+%/'(039+*2+4E+637((12+324(<,((12+E63(<,((12+326(<,

(3((,&C5C1C5+5+"!"+:&',+-%!+D-!%"-%!4+%/'(039+*2+4E+*27((61+E55(<,((91+A32(<,((61+2E2(<,

(2((,&C6C5C5+5+"!"+F&,#+-%!+D-!%"-%!4+%/'(039+*2+4E+997((36+92E(<,((36+259(<,((36+215(<,

(E((:;"CF#=C:;"CCD4C-%)-%!+$%-D$+-%!(0162+139+43+1647((36+5A6(<,((36+543(<,((36+6E9(<,

(A((,BFC:;"CCF#=C:;"C15G%+$%-D$+-%!(0162+139+4*+167((21+494(<,((21+*94(<,((23+4E4(<,

15((&#@C:;"CC,BFC:;"C15G%+$%-D$+-%!(0162+139+4*+A7((24+299(<,((24+496(<,((24+**3(<,

11((:;"C,!#-CG%CC&#@C:;"+$%-D$+-%!(0162+139+42+1*E7((26+236(<,((26+6A3(<,((26+33*(<,

14((>>"#C"!"+H!#-.&"/+IJK(0121+39+1+1697((26+*22(<,((26+3E4(<,((26+9A4(<,

16((L(L(L

19(()))*+H!#-.&"/+IJK(0121+32+45+627((22+612(<,((22+14E(<,((22+39E(<,

Figure 4.5: Traceroute from Michigan to Stanford

for packets between the two cities this was the best route on the

Internet.

Figure 4.6: Notifications of Dropped Packets

You can also see how long it took the packets to make it from the

source to each router, and then from the source to the destination.

A millisecond (ms) is a 1/1000 of a second. So 77.317 ms is just

under a tenth of a second. This network is pretty fast.

Sometimes a traceroute can take a little while, up to a minute or

two. Not all routers will give you the “I discarded your packet”

4.5. GETTING AN IP ADDRESS 45

message. In the example above, the router at hop 13 threw our

packet away without saying “I am sorry”. Traceroute waits for the

message and after a few seconds just gives up and increases the

TTL value so it gets past the rude router.

If you run a traceroute for a connection that includes an undersea

cable, you can see how fast data moves under the sea. Here

is a traceroute between the University of Michigan and Peking

University in China.

!"#$%&'$()#'"***+,-)+'.)+&/

!"#$%"&'!%(!&()))*+,'*%-'*$.(/012*034*025*03678(16(9&+:(;#<8(63(=>!%(+#$,%!:

(0((060*200*23?*242(/060*200*23?*2427((0*22@(;:((3*4@6(;:((3*452(;:

(2((AB=C.B:%=*"B=C.B:%=*';.%!*';C$9*%-'(/052*022*0@?*107((3*136(;:((3*414(;:((3*611(;:

(?((AB=C.B:%=BC2B##*;%"C!B##2*';.%!*';C$9*%-'(/052*02*@3*??7((D*400(;:((1*160(;:((1*4@@(;:

(6((052*022*0@?*?3(/052*022*0@?*?37((02*3D@(;:((1*5@5(;:((D*105(;:

(4((052*?0*55*0??(/052*?0*55*0??7((D*111(;:((@*54?(;:((0D*@10(;:

(1((052*?0*55*0D3(/052*?0*55*0D37((45*2D4(;:((45*2D?(;:((45*03@(;:

(D((0?6*D4*03@*235(/0?6*D4*03@*2357((0D?*106(;:((0D?*442(;:((0D?*???(;:

(@((0?6*D4*03D*03(/0?6*D4*03D*037((241*D13(;:(0?6*D4*03D*0@(/0?6*D4*03D*0@7((241*4D6(;:

(5((232*002*4?*0D(/232*002*4?*0D7((241*D10(;:((241*@30(;:((241*1@@(;:

03((232*002*10*04D(/232*002*10*04D7((24D*601(;:((24D*513(;:((24D*D6D(;:

00((232*002*4?*056(/232*002*4?*0567((241*@2D(;:((24D*31@(;:((241*512(;:

02((232*002*60*232(/232*002*60*2327((241*@33(;:((24D*34?(;:((241*5??(;:

Figure 4.7: Traceroute from Michigan to Peking University

You can see when the packet is encountering a long undersea

cable in steps seven and eight. The time goes from less than

1/10 of a second to nearly 1/4 of a second. Even though 1/4 of a

second is slower than 1/10 a second, it is pretty impressive when

you consider that the packet is going nearly all of the way around

the world in that 1/4 second.

The core of our IP network is remarkable. Most of the time we

don’t really care how hard the routers are working to make sure

our packets move quickly from our computer to the various des-

tinations around the world. Next we will move from looking at

how the core of the network functions to how IP addresses are

managed at the edges.

4.5 Getting an IP Address

Increasingly, computers are portable or mobile. We just pointed

out how important it was for the IP layer to track large groups

of computers using network numbers instead of tracking every

single computer individually. But since these network numbers

indicate a particular physical connection to the network, when

we move a computer from one location to another, it will need a

new IP address. Remember that the link layer address is set when

46 CHAPTER 4. INTERNETWORKING LAYER (IP)

a computer is manufactured and never changes throughout the

life of the computer. If you close your laptop in one coffee shop

and reopen it using your home WiFi, your computer will need a

different IP address.

This ability for your computer to get a different IP address when

it is moved from one network to another uses a protocol called

“Dynamic Host Configuration Protocol” (or DHCP for short). DHCP

is pretty simple. Going back to the Link layer section, recall the

first thing your computer does at the link level is ask “Is there

a base station on this network?” by sending a message to a

special broadcast address. Once your computer is successfully

connected at the link layer through that base station, it sends an-

other broadcast message, this time asking “Is there a gateway

connected to this network that can get me to the Internet? If

there is, tell me your IP address and tell me what IP address I

should use on this network”.

When the gateway router replies, your computer is given a tem-

porary IP address to use on that network (for instance, while you

are at the coffee shop). After the router has not heard from your

computer for a while, it decides you are gone and loans the IP

address to another computer.

If this process of reusing a loaned IP address goes wrong, two

computers end up on the same network with the same IP address.

Perhaps you have seen a message on your computer to the effect

of, “Another computer is using 192.168.0.5, we have stopped us-

ing this address”. Your computer sees another computer with a

link address other than its own using the IP address that your

computer thinks is assigned to it.

But most of the time this dynamic IP address assignment (DHCP)

works perfectly. You open your laptop and in a few seconds you

are connected and can use the Internet. Then you close your

laptop and go to a different location and are given a different IP

address to use at that location.

In some operating systems, when a computer connects to a net-

work, issues a DHCP request, and receives no answer, it decides

to assign itself an IP address anyway. Often these self-assigned

addresses start with “169. . . .”. When your computer has one of

these self-assigned IP addresses, it thinks it is connected to a net-

work and has an IP address, but without a gateway, it has no

possibility of getting packets routed across the local network and

onto the Internet. The best that can be done is that a few com-

puters can connect to a local network, find each other, and play

4.6. A DIFFERENT KIND OF ADDRESS REUSE 47

Figure 4.8: Getting an IP Address via DHCP

a networked game. There is not much else that can be done with

these self-assigned IP addresses.

4.6 A Different Kind of Address Reuse

If you know how to find the IP address on your laptop, you can do

a little experiment and look at the different IP addresses you get

at different locations. If you made a list of the different addresses

you received at the different locations, you might find that many

of the locations give out addresses with a prefix of “192.168.”.

This seems to be a violation of the rule that the network number

(IP address prefix) is tied to the place where the computer is con-

nected to the Internet, but a different rule applies to addresses

that start with “192.168.” (The prefix “10.” is also special).

Addresses that start with “192.168.” are called “non-routable”

addresses. This means that they will never be used as real ad-

dresses that will route data across the core of the network. They

can be used within a single local network, but not used on the

global network.

So then how is it that your computer gets an address like

“192.168.0.5” on your home network and it works perfectly

well on the overall Internet? This is because your home

48 CHAPTER 4. INTERNETWORKING LAYER (IP)

router/gateway/base station is doing something we call “Network

Address Translation”, or “NAT”. The gateway has a single routable

IP address that it is sharing across multiple workstations that are

connected to the gateway. Your computer uses its non-routable

address like “192.168.0.5” to send its packets, but as the packets

move across the gateway, the gateway replaces the address

with its actual routable address. When packets come back to

your workstation, the router puts your workstation’s non-routable

address back into the returning packets.

This approach allows us to conserve the real routable addresses

and use the same non-routable addresses over and over for work-

stations that move from one network to another.

4.7 Global IP Address Allocation

If you wanted to connect the network for a new organization

to the Internet you would need to contact an Internet Service

Provider and make a connection. Your ISP would give you a range

of IP addresses (i.e., one or more network numbers) that you

could allocate to the computers attached to your network. The

ISP assigns you network numbers by giving you a portion of the

network numbers they received from a higher-level Internet Ser-

vice Provider.

At the top level of IP address allocations are five Regional Internet

Registries (RIRs). Each of the five registries allocates IP addresses

for a major geographic area. Between the five registries, every

location in the world can be allocated a network number. The

five registries are North America (ARIN), South and Central Amer-

ica (LACNIC), Europe (RIPE NCC), Asia-Pacific (APNIC) and Africa

(AFRNIC).

When the classic IPv4 addresses like “212.78.1.25” were in-

vented, only a few thousand computers were connected to the

Internet. We never imagined then that someday we would have

a billion computers on the Internet. But today with the expansion

of the Internet and the “Internet of things” where smart cars,

refrigerators, thermostats, and even lights will need IP addresses,

we need to connect far more than a billion computers to the

Internet. To make it possible to connect all these new computers

to the Internet, engineers have designed a a new generation of

the Internet Protocol called “IPv6”. The 128-bit IPv6 addresses

are much longer than the 32-bit IPv4 addresses.

4.8. SUMMARY 49

The Regional Internet Registries (RIRs) are leading the transition

from IPv4 to IPv6. The transition from IPv4 to IPv6 will take many

years. During that time, both IPv4 and IPv6 must work seamlessly

together.

4.8 Summary

The Internetworking Protocol layer extends our network from a

single hop (Link layer) to a series of hops that result in packets

quickly and efficiently being routed from your computer to a des-

tination IP address and back to your computer. The IP layer is

designed to react and route around network outages and main-

tain near-ideal routing paths for packets moving between billions

of computers without any kind of central routing clearinghouse.

Each router learns its position within the overall network, and by

cooperating with its neighboring routers helps move packets ef-

fectively across the Internet.

The IP layer is not 100% reliable. Packets can be lost due to mo-

mentary outages or because the network is momentarily “con-

fused” about the path that a packet needs to take across the

network. Packets that your system sends later can find a quicker

route through the network and arrive before packets that your

system sent earlier.

It might seem tempting to design the IP layer so that it never loses

packets and insures that packets arrive in order, but this would

make it nearly impossible for the IP layer to handle the extreme

complexities involved in connecting so many systems.

So instead of asking too much of the IP layer, we leave the prob-

lem of packet loss and packets that arrive out of order to our next

layer up, the Transport layer.

4.9 Glossary

core router: A router that is forwarding traffic within the core of

the Internet.

DHCP: Dynamic Host Configuration Protocol. DHCP is how a

portable computer gets an IP address when it is moved to a new

location.

50 CHAPTER 4. INTERNETWORKING LAYER (IP)

edge router: A router which provides a connection between a

local network and the Internet. Equivalent to “gateway”.

Host Identifier: The portion of an IP address that is used to

identify a computer within a local area network.

IP Address: A globally assigned address that is assigned to a

computer so that it can communicate with other computers that

have IP addresses and are connected to the Internet. To simplify

routing in the core of the Internet IP addresses are broken into

Network Numbers and Host Identifiers. An example IP address

might be “212.78.1.25”.

NAT: Network Address Translation. This technique allows a single

global IP address to be shared by many computers on a single

local area network.

Network Number: The portion of an IP address that is used to

identify which local network the computer is connected to.

packet vortex: An error situation where a packet gets into an

infinite loop because of errors in routing tables.

RIR: Regional Internet Registry. The five RIRs roughly correspond

to the continents of the world and allocate IP address for the ma-

jor geographical areas of the world.

routing tables: Information maintained by each router that

keeps track of which outbound link should be used for each

network number.

Time To Live (TTL): A number that is stored in every packet

that is reduced by one as the packet passes through each router.

When the TTL reaches zero, the packet is discarded.

traceroute: A command that is available on many Linux/UNIX

systems that attempts to map the path taken by a packet as it

moves from its source to its destination. May be called “tracert”

on Windows systems.

two-connected network: A situation where there is at least two

possible paths between any pair of nodes in a network. A two-

connected network can lose any single link without losing overall

connectivity.

4.10 Questions

You can take this quiz online at http://www.net-intro.com/quiz/

4.10. QUESTIONS 51

1. What is the goal of the Internetworking layer?

a) Move packets across multiple hops from a source to destina-

tion computer

b) Move packets across a single physical connection

c) Deal with web server failover

d) Deal with encryption of sensitive data

2. How many different physical links does a typical packet cross

from its source to its destination on the Internet?

a) 1

b) 4

c) 15

d) 255

3. Which of these is an IP address?

a) 0f:2a:b3:1f:b3:1a

b) 192.168.3.14

c) www.khanacademy.com

d) @drchuck

4. Why is it necessary to move from IPv4 to IPv6?

a) Because IPv6 has smaller routing tables

b) Because IPv6 reduces the number of hops a packet must go

across

c) Because we are running out of IPv4 addresses

d) Because IPv6 addresses are chosen by network hardware

manufacturers

5. What is a network number?

a) A group of IP addresses with the same prefix

b) The GPS coordinates of a particular LAN

c) The number of hops it takes for a packet to cross the network

d) The overall delay packets experience crossing the network

6. How many computers can have addresses within network

number “218.78”?

52 CHAPTER 4. INTERNETWORKING LAYER (IP)

a) 650

b) 6500

c) 65000

d) 650000

7. How do routers determine the path taken by a packet across

the Internet?

a) The routes are controlled by the IRG (Internet Routing Group)

b) Each router looks at a packet and forwards it based on its

best guess as to the correct outbound link

c) Each router sends all packets on every outbound link (flood-

ing algorithm)

d) Each router holds on to a packet until a packet comes in from

the destination computer

8. What is a routing table?

a) A list of IP addresses mapped to link addresses

b) A list of IP addresses mapped to GPS coordinates

c) A list of network numbers mapped to GPS coordinates

d) A list of network numbers mapped to outbound links from

the router

9. How does a newly connected router fill its routing tables?

a) By consulting the IANA (Internet Assigned Numbers Author-

ity)

b) By downloading the routing RFC (Request for Comments)

c) By contacting the Internet Engineering Task Force (IETF)

d) By asking neighboring routers how they route packets

10. What does a router do when a physical link goes down?

a) Throws away all of the routing table entries for that link

b) Consults the Internet Map (IMAP) service

c) Does a Domain Name (DNS) looking for the IP address

d) Sends all the packets for that link back to the source com-

puter

11. Why is it good to have at least a “two-connected” network?

4.10. QUESTIONS 53

a) Because routing tables are much smaller

b) Because it removes the need for network numbers

c) Because it supports more IPv4 addresses

d) Because it continues to function even when a single link

goes down

12. Do all packets from a message take the same route across

the Internet?

a) Yes

b) No

13. How do routers discover new routes and improve their rout-

ing tables?

a) Each day at midnight they download a new Internet map

from IMAP

b) They periodically ask neighboring routers for their network

tables

c) They randomly discard packets to trigger error-correction

code within the Internet

d) They are given transmission speed data by destination com-

puters

14. What is the purpose of the “Time to Live” field in a packet?

a) To make sure that packets do not end up in an “infinite loop”

b) To track how many minutes it takes for a packet to get

through the network

c) To maintain a mapping between network numbers and GPS

coordinates

d) To tell the router the correct output link for a particular

packet

15. How does the “traceroute” command work?

a) It sends a series of packets with low TTL values so it can get

a picture of where the packets get dropped

b) It loads a network route from the Internet Map (IMAP)

c) It contacts a Domain Name Server to get the route for a

particular network number

54 CHAPTER 4. INTERNETWORKING LAYER (IP)

d) It asks routers to append route information to a packet as it

is routed from source to destination

16. About how long does it take for a packet to cross the Pacific

Ocean via an undersea fiber optic cable?

a) 0.0025 Seconds

b) 0.025 Seconds

c) 0.250 Seconds

d) 2.5 Seconds

17. On a WiFi network, how does a computer get an Internet-

working (IP) address?

a) Using the DHCP protocol

b) Using the DNS protocol

c) Using the HTTP protocol

d) Using the IMAP protocol

18. What is Network Address Translation (NAT)?

a) It looks up the IP address associated with text names like

“www.dr-chuck.com”

b) It allows IPv6 traffic to go across IPv4 networks

c) It looks up the best outbound link for a particular router and

network number

d) It reuses special network numbers like “192.168” across mul-

tiple network gateways at multiple locations

19. How are IP addresses and network numbers managed glob-

ally?

a) There are five top-level registries that manage network num-

bers in five geographic areas

b) IP addresses are assigned worldwide randomly in a lottery

c) IP addresses are assigned by network equipment manufac-

turers

d) IP addresses are based on GPS coordinates

20. How much larger are IPv6 addresses than IPv4 addresses?

a) They are the same size

4.10. QUESTIONS 55

b) IPv6 addresses are 50% larger than IPv4 addresses

c) IPv6 addresses are twice as large as IPv4 addresses

d) IPv6 addresses are 10 times larger than IPv4 addresses

21. What does it mean when your computer receives an IP ad-

dress that starts with “169..”?

a) Your connection to the Internet supports the Multicast proto-

col

b) The gateway is mapping your local address to a global ad-

dress using NAT

c) There was no gateway available to forward your packets to

the Internet

d) The gateway for this network is a low-speed gateway with a

small window size

22. If you were starting an Internet Service Provider in Poland,

which Regional Internet Registry (RIR) would assign you a

block of IP addresses.

a) ARIN

b) LACNIC

c) RIPE NCC

d) APNIC

e) AFRNIC

f) United Nations

56 CHAPTER 4. INTERNETWORKING LAYER (IP)