• 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 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
• Class C
128-191
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

Address 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

RFC 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

• 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

• 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

Variable 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

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
– 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

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

• Assume you are using three Class A private addresses
– 10.20.0.0
– 10.21.0.0
– 10.22.0.0
00001010.000101
00.0.0
00001010.000101
01.0.0
00001010.000101
10.0.0
• Common bits are 00001010.000101
– 8 bits in first octet + 6 bits in second octet = 14
– CIDR is 14

• Company assigned 4 contiguous Class C networks
– 200.10.10.0
11001000.00001010.00001
010.0
– 200.10.11.0
11001000.00001010.00001
011.0
– 200.10.12.0
11001000.00001010.00001
100.0
– 200.10.13.0
11001000.00001010.00001
101.0
• Summarize on common bits = 21
• Appears in routing table as 200.10.10.0/21

• Company assigned 4 contiguous Class C networks
– 200.10.101.0
11001000.00001010.11001
001.0
– 200.10.102.0
11001000.00001010.11001
010.0
– 200.10.103.0
11001000.00001010.11001
011 .0
– 200.10.104.0
11001000.00001010.11001
100.0
• Summarize on common bits = 21
• Appears in routing table as 200.10.101.0/21

• 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

• 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…..

Subnet 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

• 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

• 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

• 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

• 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

• 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.

• 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

• 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