Standard Network Mask

Binary Decimal

11111111

11111111

11111111

00000000 0

Subnet Mask

Network

Network

Network

Network

Host

240

0000 0

If you correlate the bits in the new subnet mask to a decimal number, you can see that the network mask of the subnetted network is 255.255.255.240.

If you need to understand subnetting and classless addressing, it can be useful to experiment with a subnet calculator like the one that can be found at www.subnet-calculator.com/. After a little experimentation, the principles will become relatively intuitive.

However, for many organizations' purposes, the class-based network design is usually enough to represent a logical network layout. Most small/medium organizations are capable of splitting their departments into a rough estimation of the IP class system. In larger organizations, you will find that classless IP addressing is quite common, although such organizations usually limit the network based on an IP network alignment — that is, a traditional non-routable Class A network is subnetted down with a Class C subnet mask — for example, using the 10.0.0.0 network, but splitting it up into a number of networks of the type 10.0.1.0, 10.0.2.0, and so on, each with a network mask of 255.255.255.0, and 254 possible hosts.

One thing that you should take away from this discussion of subnetting is that it is controlled on a local level. The Internet routers rarely know about how an administrator has subnetted a network because there is no way to propagate this information about the network to the whole Internet.

| r - - r Classless Interdomain Routing (CIDR) is an exception to this rule. CIDR is an interim - ~ ■"■> ■ solution to the lack of IPv4 addresses that are available. CIDR is a group of subnetted addresses that are controlled by larger organizations and have been registered by ISPs as being a domain of control. This is further subnetted by the ISP to provide a larger number of IP networks, but a lower number of hosts. Usually if you ask your ISP for a few routable IP addresses, they will give you a subnet mask as opposed to a network mask. It is up to the ISP to distinguish between the standard class-based system and the classless addressing scheme. It is unlikely that an organization would need 254 routable addresses, so ISPs can split their allocation of public addresses down to the 4, 8, or 16 addresses that you really need (which will mean 2, 6, or 14 usable addresses because one address will be the network address and one will be the broadcast address for the subnet).

Was this article helpful?

0 0

Post a comment