Supernetting??

DamienM

Hi
Can anyone explain supernetting ? I know the basics of it but just need clarification as to how you end up with your new network ID in the routing table. Cheers :P

Sartan

Supernetting... If you know variable length subnet masks, you should be able to set up your route tables just fine

Routing tables are built with CIDR with something called route summarization, which is just to put less demand on a router's processor.
It works like so:

A router is connected to campus networks:
10.163.35.0/25
10.163.35.128/25
10.163.36.0/25
10.163.36.128/25
10.163.37.0/25
10.163.37.32/28
10.163.37.64/28
10.163.37.96/28
10.163.37.128/28, etc

This is a supernet, just real quick and dirty. Assume that 10.163.0.0/24 is used to signify classrooms (3rd octet = classroom number)
Each classroom then contains 2 additional routers for left and right sides of the room. (10.163.x.0/25)

Room number 37 is a special case, it's a cisco practice lab with many networks: 1 router hosts the entire classroom, and subsequent heirarchal routers operate the practice stacks.

And an diagram that took longer than it should to make:

                                             C)10.163.35.0/25
                                            /    D)10.163.35.128/25
                                           /   /                                     
                         B)10.163.35.0/25    E)10.163.36.0/25
                       /                              /
                      /  F)10.163.36.0/25 --- G)10.136.36.128/25
                     /  /
A)10.163.0.0/16---------  H)10.163.37.0/28 -- I)10.163.37.32/28
                                    /  |        |     \
                                  /    |        |      J)10.163.37.64/28
                                 |     \      K)10.163.163.37.96/28
                                 |  L)10.163.37.96/28
                                 |
                              M)10.163.37.128/28
                              /                       \
                      N)10.163.37.132/30       O)10.163.37.136/30

Just in case that wasnt clear, heres something from Cisco's network academy:

Addresses First Second Third Fourth
192.168.98.0 11000000 10101000 01100010 00000000
192.168.99.0 11000000 10101000 01100011 00000000
192.168.100.0 11000000 10101000 01100100 00000000
192.168.101.0 11000000 10101000 01100101 00000000
192.168.102.0 11000000 10101000 01100110 00000000
192.168.105.0 11000000 10101000 01101001 00000000

Summary route is 192.168.96.0/20
192.168.96.0 11000000 10101000 01100000 00000000

Route summaries are automatically added by classless routing protocols.
Something to remember, though.. If this was EIGRP it would report the summary as a default class C netmask ~ probably not wanted.

RouterA(config)# router eigrp 1
RouterA(config-router)# network 192.168.98.0
RouterA(config-router)# network 192.168.99.0
RouterA(config-router)# network 192.168.100.0
RouterA(config-router)# network 192.168.101.0
RouterA(config-router)# network 192.168.102.0
RouterA(config-router)# network 192.168.105.0
RouterA(config-router)# no auto-summary

A few things to remember:
EIGRP doesn't do route summarization based on your vlsm without special configuration, it'll do your class A/B/C mask.
RIP v1 doesn't support subnet masks.
IGRP doesn't either.

Webmaster

Exellent explanation!

Just wanted to add "and reduce routing update traffic" to "which is just to put less demand on a router's processor." Although routing traffic is not often an issue anymore, on large super-and subnetted networks with slow WAN links and/or instable links it is still an important consideration.

DamienM

Cheers Guys. Still trying to get my head around it!! I'll get there. :P