IP Addressing: IPv4 Addressing Configuration Guide, Cisco IOS XE Release 3S

The arbitrary subdivision of network and host bits in IP address classes resulted in an inefficient allocation of IP space. For example, if your network has 16 separate physical segments you will need 16 IP network addresses. If you use 16 class B IP network addresses, you would be able to support 65,534 hosts on each of the physical segments. Your total number of supported host IP addresses is 1,048,544 (16 * 65,534 = 1,048,544). Very few network technologies can scale to having 65,534 hosts on a single network segment. Very few companies need 1,048,544 IP host addresses. This problem required the development of a new strategy that permitted the subdivision of IP network addresses into smaller groupings of IP subnetwork addresses. This strategy is known as subnetting.

If your network has 16 separate physical segments you will need 16 IP subnetwork addresses. This can be accomplished with one class B IP address. For example, start with the class B IP address of 172.16.0.0 you can reserve 4 bits from the third octet as subnet bits. This gives you 16 subnet IP addresses 24 = 16. The table below shows the IP subnets for 172.16.0.0/20.

Number

IP Subnet Addresses in Dotted Decimal

IP Subnet Addresses in Binary

05

172.16.0.0

10101100.00010000.00000000.00000000

1

172.16.16.0

10101100.00010000.00010000.00000000

2

172.16.32.0

10101100.00010000.00100000.00000000

3

172.16.48.0

10101100.00010000.00110000.00000000

4

172.16.64.0

10101100.00010000.01000000.00000000

5

172.16.80.0

10101100.00010000.01010000.00000000

6

172.16.96.0

10101100.00010000.01100000.00000000

7

172.16.112.0

10101100.00010000.01110000.00000000

8

172.16.128.0

10101100.00010000.10000000.00000000

9

172.16.144.0

10101100.00010000.10010000.00000000

10

172.16.160.0

10101100.00010000.10100000.00000000

11

172.16.176.0

10101100.00010000.10110000.00000000

12

172.16.192.0

10101100.00010000.11000000.00000000

13

172.16.208.0

10101100.00010000.11010000.00000000

14

172.16.224.0

10101100.00010000.11100000.00000000

15

172.16.240.0

10101100.00010000.11110000.00000000

When a digit that falls within the subnetwork (subnet) mask changes from 1 to 0 or 0 to 1 the subnetwork address is changed. For example, if you change 10101100.00010000.01011001.00100010/20 to 10101100.00010000.01111001.00100010/20 you have changed the network address from 172.16.89.34/20 to 172.16.121.34/20.

When a digit that falls outside the subnet mask changes from 1 to 0 or 0 to 1 the host address is changed. For example, if you change 10101100.00010000.01011001.00100010/20 to 10101100.00010000.01011001.00100011/20 you have changed the host address from 172.16.89.34/20 to 172.16.89.35/20.

To avoid having to do manual IP network, subnetwork, and host calculations, use one of the free IP subnet calculators available on the Internet.

Some people get confused about the terms network address and subnet or subnetwork addresses and when to use them. In the most general sense the term network address means “the IP address that routers use to route traffic to a specific network segment so that the intended destination IP host on that segment can receive it”. Therefore the term network address can apply to both non-subnetted and subnetted IP network addresses. When you are troubleshooting problems with forwarding traffic from a router to a specific IP network address that is actually a subnetted network address, it can help to be more specific by referring to the destination network address as a subnet network address because some routing protocols handle advertising subnet network routes differently from network routes. For example, the default behavior for RIP v2 is to automatically summarize the subnet network addresses that it is connected to their non-subnetted network addresses (172.16.32.0/24 is advertised by RIP v2 as 172.16.0.0/16) when sending routing updates to other routers. Therefore the other routers might have knowledge of the IP network addresses in the network, but not the subnetted network addresses of the IP network addresses.

The term IP address space is sometimes used to refer to a range of IP addresses. For example, “We have to allocate a new IP network address to our network because we have used all of the available IP addresses in the current IP address space”.