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VLSM and CIDR Review

VLSM and CIDR Review. CSE 421 CCNA Exploration 2 Chapter 6. Topics. Revision of classful and classless IP addressing Revision of VLSM and benefits Use of Classless Interdomain Routing (CIDR). Classful addressing. Network part and host part. Classful networks. IPv4 Address Exhaustion.

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VLSM and CIDR Review

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  1. VLSM and CIDR Review CSE 421 CCNA Exploration 2 Chapter 6 Thanks to S Ward Abingdon and Witney College

  2. Topics • Revision of classful and classless IP addressing • Revision of VLSM and benefits • Use of Classless Interdomain Routing (CIDR)

  3. Classful addressing

  4. Network part and host part

  5. Classful networks

  6. IPv4 Address Exhaustion

  7. Solutions • Long term: • Change to IP version 6.Plenty of addresses using a different scheme • Short term: • Use VLSM and CIDR to avoid wasting addresses • Use private addresses locally and NAT for internet access – lets many hosts share a few public addresses

  8. Classful Subnetting • All subnets of a main network must have the same subnet mask. • This means that they must all have the same number of hosts.

  9. ClassfulSubnetting 192.168.80.192/27 192.168.80.0/24 192.168.80.32/27 192.168.80.160/27 192.168.80.0/27 192.168.80.128/27 192.168.80.96/27 192.168.80.64/27

  10. Subnet 192.168.1.0/24 10 hosts • Need 6 networks, up to 26 hosts. • Borrow 3 bits, /27, 255.255.255.224 • Gives 8 networks, up to 30 hosts. • Point to point need 2. 28x3 = 84 wasted 26 hosts 12 hosts

  11. Subnet 172.16.0.0/16 100 hosts • Need 6 networks, up to 500 hosts. • Borrow 7 bits, /23, 255.255.254.0 • Gives 128 networks, up to 510 hosts. • Point to point need 2. 508x3 = 1524 wasted 500 hosts 350 hosts

  12. Waste • Classful subnetting wastes addresses. • If you are using private addresses then you may not be bothered. • Waste of public addresses does matter.

  13. Need 10 addresses /28. Give them 16. Need 500 addresses /23. Give them 512. Solution: • Variable Length Subnet Masking (VLSM) • Create subnets as per specific host requirements.

  14. Variable Length Subnet Masking (VLSM) “If you know how to subnet, you can do VLSM.” What’s the trick? Always satisfy the requirements of your biggest LAN and then work your way down ….

  15. Variable Length Subnet Masking (VLSM)

  16. VLSM 201.1.0.0/22 Advertise? 201.1.4.0/23 Router A 201.1.6.0/24 How many entries does Router A has in it’s routing table? 201.1.7.0/24

  17. Route summarization Octet 3 in binary 00000000000001000000011000000111 • 201.1.0.0/22 • 201.1.4.0/23 • 201.1.6.0/24 • 201.1.7.0/24 Same Difference starts here Same Difference starts here 21 bits the same so use /21 for summary

  18. Route summarization 201.1.0.0/22 Advertise201.1.0.0/21 201.1.4.0/23 201.1.6.0/24 • A Supernetsummarizes multiple network addresses with a mask that is less than (or a summary of) the classful mask. 201.1.7.0/24

  19. Octet 2 in binary 000100000001000100010001 7 the same here Route summarisation • What address would summarise: • 170.16.0.0/16 • 170.17.0.0/17 • 170.17.128.0/17 • 15 the same altogether • 170.16.0.0/15 You can create static summary routes. Dynamic routes can be summarised.

  20. CIDR Concept • Classless InterDomain Routing (CIDR) introduced 1993 by IETF. • To CIDR-compliant routers, address class is meaningless. • The network portion of the address is determined by the network subnet mask, also known as the network prefix, or prefix length (/8, /19, etc.).

  21. Classless Routing Protocols • Classless routing protocols are complaint with CIDR which means they understand • VLSM and • Route Summarization. • All routing protocols are classless nowadays.

  22. VLSM Exercise • Subnet 172.16.0.0/16 by borrowing 4 bits. • Then subnet the third subnet by borrowing 2 more bits. • Write out the subnet addresses and masks.

  23. Subnetting 172.16.0.0/16

  24. The End

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