From Subnettingto VLSM

From Subnettingto VLSM

Classful vs. Classless Routing

VLSM Explained

Why VLSM

Suggestions for Teaching VLSM

Credits

• Virginia Phillips, CCNA, CCAI– Instructor CCNP classes, Youngstown State

University

• Edmund Ickert, CCNA, CCAI– Instructor CCNA classes, Youngstown State

University, completed all CCNP courses

• Sandeep Kolwalkar, CCNA– Graduate Student, taking CCNP classes,

Youngstown State University

Classful vs Classless Routing

• Classful routing assigns address space based on the value in the first octet of the 32-bit IP address– RFC Number 791 (760)– Class based on value in first octet value– Receiving router ands subnet mask to determine subnet

• Class A 0-126• Class B 128-191• Class C 192-223

• Classless routing ignores classes and uses a CIDR value (number of 1s in network mask) to identify the network– CIDR transmitted as part of IP address – RFC 1517-1520– Network portion not restricted to entire octet

Classless RoutingAddress Space Issues

• Class A and Class B = 75% address space– < 17000 organizations can be assigned address

• Class C = 12.5% available address space– Each network limited to 254 maximum hosts– Potential routing problems

• Too many network addresses in routing table• Extra work for CPU; more memory required

Private AddressingRFC 1918

• Class A 10.0.0.0 to 10.255.255.255

• Class B 172.16.0.0 to 172.31.255.255

• Class C 192.168.0.0 to 192.168.255.255– Used to extend life of IPv4 addressing– Note: Do not mix private and public IP address in

same network – it will create discontiguous subnets which causes problems

Classless Routing

• Another method used to extend the life of IPv4• Temporary solution to deal with lack of network

numbers • Uses bit mask (NOT 1st octet value) to determine

network portion of address• Uses CIDR to summarize routing information;

CIDR transmitted with IP address• Enables the use of supernets and/or route

aggregation and summarization– Smaller routing tables– Reduced router memory requirements– Reduced number of CPU cycles for routing processes

Routing Protocols

• Classful – can’t send subnet information in updates– RipV1, IGRP, EGP, BGP3 – also can’t support discontiguous

subnets

• Classless – Sends CIDR in updates sent via multicasting– Can authenticate

• RipV2 (RFC 1058), EIGRP, OSPF, IS-IS, BGP4– RIPV2 and EIGRP automatically summarize at classful boundary

unless you configure differently» RouterA (config-router) no auto-summary

VLSMVariable Length Subnet Masking

• Subnets a subnet

• Can support multiple contiguous routes

• Can use more than one subnet mask for address space allocated to a firm

• Makes more efficient use of available address space– Creates two-host subnets for serial links

Why Not IPv6?128-bit address space

• Slow to arrive

• IPv4 revitalized with new features– VLSM, NAT/PAT, IP unnumbered, private

addresses

• Not supported by legacy systems

• Requires new software (and hardware)

• Requires retraining

Zero Subnet (Ones too?)

• Zero subnet– IOS 12.X and higher supports by default– Configure pre-12.x IOS routers

• RouterA(config) IP subnet-zero

– DO Use it to increase address space available

• Ones subnet– Defined in RFC 1878– Can use it; however can cause problems – Avoid using unless you absolutely need it

Route Aggregation Example 1

• Assume you are using three Class B private addresses– 172.16.0.0 10101100.000100 00.0.0– 172.17.0.0 10101100.000100 01.0.0– 172.18.0.0 10101100.000100 10.0.0

• Common bits are 10111000.0001– 8 bits in first octet + 6 bits in second octet = 14– CIDR is 14

• Insulates upstream routers from route flapping problems (serial link problem)

Route Aggregation Example 2

• Assume you are using three Class A private addresses– 10.20.0.0 00001010.000101 00.0.0– 10.21.0.0 00001010.000101 01.0.0– 10.22.0.0 00001010.000101 10.0.0

• Common bits are 00001010.000101– 8 bits in first octet + 6 bits in second octet = 14– CIDR is 14

Supernet Example 1

• Company assigned 4 contiguous Class C networks– 200.10.10.0 11001000.00001010.00001010.0– 200.10.11.0 11001000.00001010.00001011.0– 200.10.12.0 11001000.00001010.00001100.0– 200.10.13.0 11001000.00001010.00001101.0

• Summarize on common bits = 21

• Appears in routing table as 200.10.10.0/21

Supernet Example 2

• Company assigned 4 contiguous Class C networks– 200.10.101.0 11001000.00001010.11001001.0– 200.10.102.0 11001000.00001010.11001010.0– 200.10.103.0 11001000.00001010.11001011.0– 200.10.104.0 11001000.00001010.11001100.0

• Summarize on common bits = 21

• Appears in routing table as 200.10.101.0/21

Network Subnet Example

• 128.1.0.0/16 is assigned IP address– 130 subnets needed – Requires use of third octet for subnet values

• 1,2,3,4, …., 254

– Each subnet can support 254 hosts– Each serial connection will use a subnet and waste

252 address spaces

Network Subnet Example

• Assigned IP address is 128.1.0.0– Scenario - 130 subnets needed and 20 serial

connections used now– Requires use of third octet for subnets

• 128.1.0.0 to 128.1.254.0, subnet mask 255.255.255.0 or CIDR 24

• Each subnet can support 254 hosts• To use an entire subnet for a serial connection would

waste 252 address spaces and we have 20 now – SO…..

Network Subnet ExampleSubnet the Subnet

• Use subnets 128.1.0.0 to 128.1.129.0 for needed subnets with a CIDR of 24

• Subnet subnet 128.1.130.0 using CIDR 30– 128.1.130.0/30– 128.1.130.4/30– 128.1.130.8/30– ………………..– 128.1.130.252/30

Network 2 Subnet Example

• A Network address of 200.10.20.0 is assigned– Subnet with a CIDR of 26

• 200.10.20.0, 200.10.20.64 (62 hosts)

– Subnet subnet 128 with a CIDR of 28• 200.10.20.128, 200.10.20.144, 200.10.20.160 (14 hosts)

– Subnet subnet 200.10.20.176 with a CIDR of 30• 200.10.20.176, 200.10.20.180, 200.10.20.184 (2 hosts)

• Can summarize (aggregate) on– 200.10.20.0/26

Using VLSM

• Variable Length Subnet Masking – allows division of address space based on the size of networks – Start with network requiring the most addresses– Create a subnet mask (use CIDR – Classless

InterDomain Routing – number)– Subnet the subnet as needed to provide address

space required for other subnets• Be logical – start at beginning or end or address space• Addresses must be contiguous to enable route

summarization

Teaching Tips 1

• Make certain students understand subnetting– Provide students with a mix of subnetting problems

using Class A, B, and C addresses and different numbers of bits borrowed to ensure they do understand

• Show relationship of CIDR number of subnet mask

Teaching Tips 2

• Explain reasons for using VLSM

• Explain route aggregation (summarization)

• Explain supernetting

• Show how to summarize using common bits

• Show how to supernet using common bits

Teaching Tips 3

• Show a simple VLSM example using the third octet– First subnet for 255 subnets with 254 hosts;

CIDR = 24– Then subnet one of the subnets for subnets

with CIDR of 28• Subnet 200.16, 200.32, 200.48, etc.

– Then subnet one of the subnets for subnets to use for serial lines and a CIDR of 30

• Subnet 201.4, 201.8, 201.12, 201.16, etc.

Teaching Tips 4

• Show a second example using the fourth octet– Subnet for 8 subnets with a CIDR of 27

• Subnets 0, 32, 64, 96, 128, 160, 192, 224

– Subnet subnet 96, 128, and 160 with a CIDR of 28

• Subnets 96, 112, 128, 144, 160, 176

– Subnet subnets 192 and 224 with a CIDR of 30• Subnets 192, 196, 200, 204, 208, 212, 216, 220, 224,

228, 232, 236, 240, 244, 248, 252

Teaching Tips 5

• Show examples of divided address spaces– Do not use slides – use hard copy and give

students a copy

• Give several problems moving from a very simple problem to a very complex problem– Provide answers for each problem for students to

check as problem is completed