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Subnet


Explanation

A Subnet is a subdivision of a TCP/IP network.

Subnets can be large or small, but generally, smaller subnets make for more efficient Routing.

As an example, say you had a private TCP/IP network with the address range:
192.168.0.0 - 192.168.255.255. This gives you 65536 addresses. If you had that many devices on your network you would probably be swamped with Broadcast traffic and things would slow to a crawl.

Your network or subnet is defined by its Subnet Mask or Netmask.

In the example above your netmask is
255.255.0.0 - in Windows terminology
192.168.0.0/16 - in Unix terminology

The parts in bold are the actual netmask specification.


The first IP address of your subnet is always the Network Address and the last IP address is almost always the Broadcast Address. These are special addresses reserved by the TCP/IP protocol for doing special things. That means you always have two less IP addresses that you can use for your devices than what your netmask specifies.

In the above example the Network address is 192.168.0.0 and the Broadcast Address is 192.168.255.255. That leaves you with 65534 addresses.

Suppose you wanted to subdivide a part off of the very large subnet in the above example. You could start your new subnet with the address 192.168.1.0. If you wanted 256 addresses your netmask would be:

255.255.255.0 or 192.168.1.0/24.

If you wanted another, even smaller subnet somewhere else in the larger address range above, say 16 addresses, and you wanted the address
192.168.70.45 to be within that address range (for whatever reason).

Your netmask would be
255.255.255.240 or 192.168.70.32/28

192.168.70.32 would be your Network Address
192.168.70.47 would be your Broadcast Address

Calculators

These numbers were calculated using the online subnet calculator at External linkhttp://www.telusplanet.net/public/sparkman/netcalc.htm.
There are also various free subnet calculators for Windows and Linux downloadable from the net. External linkGoogle will find them for you.

If you run the External linkShorewall firewall available for Linux then an ipcalc command has been available since version 1.4.6
     ipcalc [ <address> <netmask> | <address>/<vlsm> ]

Example:
     root@wookie:# shorewall ipcalc 10.10.144.48/28
       CIDR=10.10.144.48/28
       NETMASK=255.255.255.240
       NETWORK=10.10.144.48
       BROADCAST=10.10.144.63

Melbourne Wireless Subnets

From LocFinder, Melbourne Wireless nodes are allocated subnets from the 10.10.0.0/16 address range. Upon request, individual nodes are given /28 subnets giving them 14 usable addresses per subnet. In the near future, nodes may request smaller subnets if they wish.


Version 5 (current) modified Tue, 03 Jul 2007 23:12:20 +1000 by graybeard
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