CHAPTER 6 Part 21

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ROUTE SUMMARIZATION (AGGREGATION) When advertising routes into another major

network, classful routing protocols automatically summarize subnets. Classful only advertise a route to a Class A,B and C

network, instead of routes to subnets. Classful routers and hosts do no undertand nonlogical

prefix length and subnet. Why summarizing?

Reduce the size of routing table which minimizes bandwidth consumption and processing on routers.

It also able to keep the problem within one area of the network from spreading to other areas.

The automatic summarization into a major class network has disadvantages – discontiguous subnet is not supported. 2

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SUPERNETTING/EXAMPLE OF ROUTE SUMMARIZATION

The network administrator assigned network numbers 172.16.0.0 through 172.19.0.0 to networks in a branch office.

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172.16.0.0

172.17.0.0

172.18.0.0

172.19.0.0

Branch-Office Networks

Enterprise Core Network

Branch-Office Router

CONTINUE..

The branch office can summarize its local network numbers and report that it can reach 172.16.0.0/14.

By advertising this single route, the router is saying” route packets to me if the destinations has the first 14 bits set to 172.16- the first 14 bits are equal to 10101100000100

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172.16.0.0/14 SUMMARIZATION

Second Octet in Decimal

Second Octet in Binary

16 00010000

17 00010001

18 00010010

19 00010011

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• the leftmost 6 bits from 16 19 are identical.

ROUTE SUMMARIZATION TIPS

Multiple IP addresses must share the same leftmost bits

Routers must base their routing decisions on a 32-bit IP address and prefix length that can be up to 32 bits

Routing protocols must carry the prefix length with 32-bit addresses.

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CONTINUE..

Additions:When looking at a block of subnets, you can

determine if the addresses can be summarized by the following rules: The number of subnets to be summarized must

be must be a power of 2 (2,4,8 etc) The relevant octet in the first address in the

block to be summarized must be a multiple of the number of subnets

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EXAMPLE

The following network numbers are defined at branch office. Can they be summarized? 192.168.32.0 192.168.33.0 192.168.34.0 192.168.35.0 192.168.36.0

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DISCONTIGUOUS SUBNETS

• based on the above figure, both routers cannot reach remote subnets of network 10.0.0.0 since there are not connected. • solve it suing CIDR – a classless routing protocol. •Router A advertises that it can get to network 10.108.16.0/20. •Router B advertises that it can get to network 10.108.32.0/20•CIDR understand prefixes of any length , the routers can route to DS.

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GUIDELINES FOR ASSIGNING NAMES Names should be

Short Meaningful Unambiguous Distinct Case insensitive

Avoid names with unusual characters Hyphens, underscores, asterisks, and so on

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DOMAIN NAME SYSTEM (DNS)

Is a distributed database, supports hierarchical naming Has 2 parts: a hostname and a domain name.

example: information.priscilla.com

Maps names to IP addresses

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CONTINUE..

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DNS DETAILS

Client/server modelClient is configured with the IP

address of a DNS server Manually or DHCP can provide the

addressDNS resolver software on the

client machine sends a query to the DNS server. Client may ask for recursive lookup.

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DNS RECURSION

A DNS server may offer recursion, which allows the server to ask other servers Each server is configured with the IP address of

one or more root DNS servers.

When a DNS server receives a response from another server, it replies to the resolver client software. The server also caches the information for future requests. The network administrator of the authoritative

DNS server for a name defines the length of time that a non-authoritative server may cache information. 14

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SUMMARY

Use a systematic, structured, top-down approach to addressing and naming

Assign addresses in a hierarchical fashion Distribute authority for addressing and

naming where appropriate IPv6 looms in our future

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REVIEW QUESTIONS

Why is it important to use a structured model for addressing and naming?

When is it appropriate to use IP private addressing versus public addressing?

When is it appropriate to use static versus dynamic addressing?

What are some approaches to upgrading to IPv6?

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