This document discusses IP addressing and subnetting. It provides examples of subnetting different size networks using various subnet masks. It also works through an example of allocating IP addresses across multiple LANs and WANs with varying size requirements. The example highlights the need for VLSM to efficiently allocate addresses while allowing for future expansion without redesign.

1. IP Addressing/Subnetting
Simplified
Brian Sterck
May 6, 2005

2. Class A
Subnetting
Options

3. Class B
Subnetting
Options

4. Class C
Subnetting
Options

5. Brain Benders

6. More Tools…

7. Practical Example

8. Do we have enough addresses?
• How many addresses have been
allocated?
• How many are needed?

9. Address Allocation
• Total Address Needed:
LANs = 14+10+19+23+6+17+14 = 103
WANs = 4+4+4+4+4+4 = 24
Total = 103 + 24 = 127

10. Understanding IP Requirements
• Subnetworks on the LAN/WAN need to be
issued using perfect powers of 2
• 22 = 4
• 23 = 8
• 24 = 16
• 25 = 32
• 26 = 64
• 27 = 128

11. Scaling Subnets
• With a requirement of 23 nodes…
• The next perfect power of 2 that meets
this need = 32
• 25 = 32 5 host bits needed
• Last Octet is then represented as:
– A.B.C.NNNHHHHH N=Network H=Host
– 11111111.11111111.11111111.11100000

12. Subnetting Examples
SubnetMask SubnetMask # Hosts
255.255.255.0 /24 256 (254)
255.255.255.128 /25 128 (126)
255.255.255.192 /26 64 (62)
255.255.255.224 /27 32 (30)
255.255.255.240 /28 16 (14)
255.255.255.248 /29 8 (6)
255.255.255.252 /30 4 (2)

13. Practical Example

14. Address Allocation
• Total Addresses Needed:
LANs = 16+16+32+32+8+32+16 = 152
= /28 + /28 + /27 + /27 + /29 + /27 + /28 = 152
WANs = 4+4+4+4+4+4 = 24
= /30 + /30 + /30 + /30 + /30 + /30 = 24
Total = 103 + 52 = 157

15. Reference Points
• Use numbers, symbols, letters to represent
each LAN / WAN link
• Use some consistent method for assignment
– Size of Subnetwork
– Location
– Zone or Distribution Point

16. Assign Reference Points

17. Subnetting Options

18. Subnetting Options (cont.)

19. VLSM and Routing Protocols
• Does your routing protocol support varying
the length of the subnet mask from one
interface to another?
• Can you re-subnet a subnet differently
from one interface to another and still
advertise that subnetwork via RIPv1,
RIPv2, OSPF, EIGRP, ISIS, BGP?

20. Routing Protocols
• Supporting VLSM
– RIP version 2
– EIGRP
– OSPF
– ISIS
• Not Supporting VLSM
– IGRP
– RIP version 1

21. Which Routing Protocols will
advertise all Networks correctly?

22. Non-VLSM capable Routing
Protocols will work just fine!

24. Problem!
• The 192.168.20.0 /24 network gives us:
– 1 Network
– 254 Nodes Available for Assignment
• 13 Subnetworks Required
• Network Demands range from 4 – 32
nodes per LAN / WAN segment

25. Start Subnetting

26. Subnetting

27. Subnetting

28. Box Diagram Alternate

29. Address Range

51. Future Expansion Danger
• Note the three locations with the and
• If we need to add a network printer or visitor
with laptop, the IP addressing scheme will have
to be modified.
• This results in downtime and careful migration
planning
There must be a better way!