supernetting

vandita · Joined: 27 Jul 2004 Posts: 1

can any one tell me the basics of supernetting and the important rules for it.

suppose i ve to combine 3 networks,
199.199.127.0
199.199.128.0
199.199.129.0

can i combine the above three addresses?

if yes then how and if no then why?

jackson · Joined: 14 Nov 2004 Posts: 1

Hi vandita, supernetting is basically used to summarize multiple network addresses into one network address, the resulting address will be advertised on the router along with it's corresponding bitmask, this process saves space in the routing table entries.

Unfortunately the three networks you spacified can not be summarized, why? because one of the rules of supernetting is that the block size must be restricted to the power of 2, therefore in this scenario we will have 4 networks per block, that’s because you want to summarize 3 networks (199.199.127.0, 199.199.128.0 and 199.199.129.0) so the closest number to 3 is 2^2=4 which is >= 3 (we don’t need to subtract 2 for host and network in supernetting). so the problem is that network 199.199.127.0 falls in a block that is just before the block that contains 199.199.128.0 and .129.0

First we set the first 2 bits in the third octet to 0`s, this will allow us to summarize 4 networks (2^2=4)
255 . 255 . 252 . 0
Bit Mask : 11111111.11111111.111111(00).00000000

now we just start with the first block (assuming your allowed to use network 0 if not, then start from the next one)
199 . 199 . 0 - 3 . 0
11000111.11000111.000000(00).00000000
11000111.11000111.000000(01).00000000
11000111.11000111.000000(10).00000000
11000111.11000111.000000(11).00000000

second block is:
199 . 199 . 4 - 7 . 0
11000111.11000111.000001(00).00000000
11000111.11000111.000001(01).00000000
11000111.11000111.000001(10).00000000
11000111.11000111.000001(11).00000000

the process continues until we reach network 124
199 . 199 . 124 - 127. 0
11000111.11000111.011111(00).00000000
11000111.11000111.011111(01).00000000
11000111.11000111.011111(10).00000000
11000111.11000111.011111(11).00000000

notice here that .127 is part of the above block, now lets look at the block right after that one
199 . 199 . 128 - 131 . 0
11000111.11000111.100000(00).00000000
11000111.11000111.100000(01).00000000
11000111.11000111.100000(10).00000000
11000111.11000111.100000(11).00000000

its clear that network 199.199.127.0 belongs to a different block to that of 199.199.128.0 and 199.199.129.0, the addresses that you want to summerize must (1) be in sequence and (2) the addresses must fall in the same block. supernetting only works when these conditions are satisfied.

Some usefull links:
http://www.subnet-calculator.com/supernets.php
http://www.red.net/support/resourcecentre/leasedline/intro.php

i hope that helped

alpacino · Joined: 28 May 2005 Posts: 1

Currently, we are upgrading bandwidth between our different physical sites. At present, we have five physical sites/subnets linked to our HQ site via leased line.

After the upgrade of the bandwidth, three of the sites will be linked to our HQ site via fibre and two of the sites will be linked to our HQ using leased lines.

Currently, the IP Addressing is as follows.

Site A (HQ) 172.16.1.0 (Class C Subnet Mask 255.255.255.0)
Site B 172.16.2.0 (Class C Subnet Mask 255.255.255.0)
Site C 172.16.3.0 (Class C Subnet Mask 255.255.255.0)
Site D 172.16.4.0 (Class C Subnet Mask 255.255.255.0)
Site E 172.16.5.0 (Class C Subnet Mask 255.255.255.0)
Site F 172.16.6.0 (Class C Subnet Mask 255.255.255.0)

The sites E and F will still be connected to Site A using leased lines. These three sites will have a router.

The sites B, C and D will be connected to the Site A (HQ) via fibre link. These three sites will be on the same LAN as the Site A. Because they will become on the same LAN, they will not require a router.

However, this will create a problem of running out of IP Addresses at the Site A. Having read a few articles I believe that supernetting can solve my problem.

The problem is I don't know how I can supernet these sites and which subnet mask I should use. Also, how will my DHCP server allocate the IP Addresses?

Please help.