Networking exercise

Classes

The following are the classes of IP addresses.

 Class A—The first octet denotes the network address, and the last three octets are the host portion. Any IP address whose first octet is between 1 and 126 is a Class A address. Note that 0 is reserved as a part of the default address, and 127 is reserved for internal loopback testing.

1-126

 Class B—The first two octets denote the network address, and the last two octets are the host portion. Any address whose first octet is in the range 128 to 191 is a Class B address.

128-191

 Class C—The first three octets denote the network address, and the last octet is the host portion. The first octet range of 192 to 223 is a Class C address.

192-223

 Class D—Used for multicast. Multicast IP addresses have their first octets in the range 224 to 239.

224-239 (Multicast)

 Class E—Reserved for future use and includes the range of addresses with a first octet from 240 to 255.

240-255 (Reserved)

Subnetting

Subnetting is the concept of dividing the network into smaller portions called subnets. This is done by borrowing bits from the host portion of the IP address, enabling more efficient use of the network address. A subnet mask

defines which portion of the address is used to identify the network and which denotes the hosts.

The following tables show all possible ways a major network can be subnetted, and, in each case, how many effective subnets and hosts are possible.

There are three tables, one for each class of addresses.

 The first column shows how many bits are borrowed from the host portion of the address for subnetting.

 The second column shows the resulting subnet mask in dotted decimal format.

 The third column shows how many subnets are possible.

 The fourth column shows how many valid hosts are possible on each of these subnets.

 The fifth column shows the number of subnet mask bits.

Class A Host/Subnet Table

Class A

Number of

Bits Borrowed Subnet Effective Number of Number of Subnet

from Host Portion Mask Subnets Hosts/Subnet Mask Bits

------- --------------- --------- ------------- -------------

1 255.128.0.0 2 8388606 /9

2 255.192.0.0 4 4194302 /10

3 255.224.0.0 8 2097150 /11

4 255.240.0.0 16 1048574 /12

5 255.248.0.0 32 524286 /13

6 255.252.0.0 64 262142 /14

7 255.254.0.0 128 131070 /15

8 255.255.0.0 256 65534 /16

9 255.255.128.0 512 32766 /17

10 255.255.192.0 1024 16382 /18

11 255.255.224.0 2048 8190 /19

12 255.255.240.0 4096 4094 /20

13 255.255.248.0 8192 2046 /21

14 255.255.252.0 16384 1022 /22

15 255.255.254.0 32768 510 /23

16 255.255.255.0 65536 254 /24

17 255.255.255.128 131072 126 /25

18 255.255.255.192 262144 62 /26

19 255.255.255.224 524288 30 /27

20 255.255.255.240 1048576 14 /28

21 255.255.255.248 2097152 6 /29

22 255.255.255.252 4194304 2 /30

23 255.255.255.254 8388608 2* /31

Class B Host/Subnet Table

Class B Subnet Effective Effective Number of Subnet

Bits Mask Subnets Hosts Mask Bits

------- --------------- --------- --------- -------------

1 255.255.128.0 2 32766 /17

2 255.255.192.0 4 16382 /18

3 255.255.224.0 8 8190 /19

4 255.255.240.0 16 4094 /20

5 255.255.248.0 32 2046 /21

6 255.255.252.0 64 1022 /22

7 255.255.254.0 128 510 /23

8 255.255.255.0 256 254 /24

9 255.255.255.128 512 126 /25

10 255.255.255.192 1024 62 /26

11 255.255.255.224 2048 30 /27

12 255.255.255.240 4096 14 /28

13 255.255.255.248 8192 6 /29

14 255.255.255.252 16384 2 /30

Class C Host/Subnet Table

Class C Subnet Effective Effective Number of Subnet

Bits Mask Subnets Hosts Mask Bits

------- --------------- --------- --------- --------------

1 255.255.255.128 2 126 /25

2 255.255.255.192 4 62 /26

3 255.255.255.224 8 30 /27

4 255.255.255.240 16 14 /28

5 255.255.255.248 32 6 /29

6 255.255.255.252 64 2 /30

Subnetting Example

The first entry in the Class A table (/10 subnet mask) borrows two bits (the leftmost bits) from the host portion of the network for subnetting, then with two bits you have four (2

2 ) combinations, 00, 01, 10, and 11. Each of these will

represent a subnet.

Binary Notation Decimal Notation

-------------------------------------------------- -----------------

xxxx xxxx. 0000 0000.0000 0000.0000 0000/10 ------> X.0.0.0/10

xxxx xxxx. 0100 0000.0000 0000.0000 0000/10 ------> X.64.0.0/10

xxxx xxxx. 1000 0000.0000 0000.0000 0000/10 ------> X.128.0.0/10

xxxx xxxx. 1100 0000.0000 0000.0000 0000/10 ------> X.192.0.0/10

Out of these four subnets, 00 and 11 are called subnet zero and the all-ones subnet, respectively. Prior to Cisco IOS

® Software Release 12.0, the ip subnet-zero global configuration command was required to be able to configure

subnet zero on an interface. In Cisco IOS 12.0, ip subnet-zero is enabled by default.

Note: The subnet zero and all-ones subnet are included in the effective number of subnets as shown in the third column.

Since the host portion has now lost two bits, the host portion will have only 22 bits (out of the last three octets). This means the complete Class A network is now divided (or subnetted) into four subnets, and each subnet can have 2

22 hosts (4194304). A host portion with all zeros is network number itself, and a host portion with all ones is reserved

for broadcast on that subnet, leaving the effective number of hosts to 4194302 (2 22

– 2), as shown in the fourth column.