Post on 25-Dec-2015
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