1. From Subnetting to VLSM
Classful vs. Classless Routing
VLSM Explained
Why VLSM
Suggestions for Teaching VLSM

2. Credits Virginia Phillips, CCNA, CCAI Edmund Ickert, 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

3. 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
Class C
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
Network portion not restricted to entire octet

4. Classless Routing Address Space Issues
Class A and Class B = 75% address space
< 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

5. Private Addressing RFC 1918
Class A to
Class B to
Class C to
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

6. 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

7. 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

8. VLSM 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

9. 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

10. Zero Subnet (Ones too?) Zero subnet Ones 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

11. Route Aggregation Example 1
Assume you are using three Class B private addresses
Common bits are
8 bits in first octet + 6 bits in second octet = 14
CIDR is 14
Insulates upstream routers from route flapping problems (serial link problem)

12. Route Aggregation Example 2
Assume you are using three Class A private addresses
Common bits are
8 bits in first octet + 6 bits in second octet = 14
CIDR is 14

13. Supernet Example 1 Company assigned 4 contiguous Class C networks
Summarize on common bits = 21
Appears in routing table as /21

14. Supernet Example 2 Company assigned 4 contiguous Class C networks
Summarize on common bits = 21
Appears in routing table as /21

15. Network Subnet Example
/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

16. Network Subnet Example
Assigned IP address is
Scenario subnets needed and 20 serial connections used now
Requires use of third octet for subnets
to , subnet mask 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…..

17. Network Subnet Example Subnet the Subnet
Use subnets to for needed subnets with a CIDR of 24
Subnet subnet using CIDR 30
/30
/30
/30
………………..
/30

18. Network 2 Subnet Example
A Network address of is assigned
Subnet with a CIDR of 26
, (62 hosts)
Subnet subnet 128 with a CIDR of 28
, , (14 hosts)
Subnet subnet with a CIDR of 30
, , (2 hosts)
Can summarize (aggregate) on
/26

19. 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

20. 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

21. 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

22. 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 , , , etc.
Then subnet one of the subnets for subnets to use for serial lines and a CIDR of 30
Subnet 201.4, 201.8, , , etc.

23. 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

24. 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