IP Subnetting

As it turns out, dividing IP addresses into classes A, B and C is not flexible enough. In particular, it does not make efficient use of the available IP addresses and it does not give network administrators enough control over their internal LAN configurations.

In this diagram, the class B network 131.108 is split (probably into 256 subnets), and a router connects the 131.108.2 subnet to the 131.108.3 subnet.

IP Subnet Mask

A subnet mask tells a computer or a router how to divide a range of IP addresses into the network part and the host part.


Address =

Subnet Mask =

Subnet =

In this example, without a subnet mask the address would be treated as class B and the network number would be 131.108. But because someone supplied a subnet mask of, the network number is actually 131.108.2.

These days, routers and computers always use subnet masks if they are supplied. If there is no subnet mask for an address, then the class A, B, C scheme is used.

Remember that a network mask determines which portion of an IP address identifies the network and which portion identifies the host, while a subnet mask describes which portion of an address refers to the subnet and which part refers to the host.

IP Address Assignment

  •  – ISPs assign addresses to customers
  •  – IANA assigns addresses to ISPs
  •  – CIDR block: bundle of addresses

Historically, an organization was assigned a class A, B or C address and carried that address around. This is no longer the case.

Usually an organization is assigned IP addresses by its ISP. If an organization changes ISPs, it changes IP addresses. This is usually not a problem, since most people refer to IP addresses using the DNS. For example, www.acme.com might point to today and point to tomorrow, but nobody other than the system administrator at acme.com has to worry about it.

IANA—the Internet Assigned Numbers Authority—assigns IP addresses to ISPs. These days no one gets a class A or a class B network—they are pretty much all gone. Usually the IANA bundles 8 or 16 or 32 class C networks together and calls it a CIDR (pronounced “cider”) block. CIDR stands for Class Independent Routing, and it greatly simplifies routing among the Internet backbones. CIDR blocks are sometimes called supernets (as opposed to subnets).

IPv6 Addressing

  • – 128-bit addresses
  • – 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses

Example1:- 5F1B:DF00:CE3E:E200:0020:0800:5AFC:2B36
Example2:- 0:0:0:0:0:0:

With the explosive growth of the Internet, there are not enough IPv4 addresses to go around. IPv6 is now released, and many organizations are already migrating.

While IPv6 has a number of nice features, its biggest claim to fame is a huge number of IP addresses. IPv4 was only 32 bits; IPv6 is 128 bits.

To ease migration, IPv6 completely contains all of IPv4, as shown in the second example above.

Most network applications will have to be modified slightly to accommodate IPv6.


  • TCP/IP is a suite of protocols
  • TCP/IP defines communications between computers on the Internet
  • IP determines where packets are routed based on their destination address
  • TCP ensures packets arrive correctly at their destination address
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