TAILIEUCHUNG - The Complete IS-IS Routing Protocol- Part 12

The Complete IS-IS Routing Protocol- P12: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | IS-IS Addressing 97 is a fixed 32-bit entity. It has a different meaning for routers and for hosts like PCs or workstations. Please note that the term hosts in this context has nothing to do with mainframe technology. It is simply a term borrowed from ancient IP terminology meaning a computer or workstation that runs IP. For a host the address consists of two parts The network part and the host part. The boundary between the network and host part is defined using the network mask. The network mask is typically a continguous sequence of bits usually written down in decimal notation. For instance a 24-bit netmask could be written as 24 consecutive bits or in more readable decimal representation as . Since the introduction of classless interdomain routing CIDR in the Internet as described in RFC 1518 it has become common not to write up the entire netmask but just the prefix-length. The prefix-length is the decimal representation of the bit border between the network and the host part of the IP address. The shorter the prefixlength the larger the host count beyond. Table shows the relationship between a few selected prefix lengths netmasks and the potential host count. Why is the netmask or prefix-length important Because routers and hosts figure out based on the prefix-length or netmask if a destination address is on the local sub-net or not. If the prefixes of the source and destination match then the stations are on the same sub-net or at least they should be for IP to work properly . Consider the example in Figure which shows IP address 24. The trailing 24 indicates a network hosts border at 24 bits. The router and other hosts applies the netmask to find out if a given source destination address pair in a packet is on the same sub-net. If this example host wants to communicate with host . Each IP device knows that the destination host is local. How The two IP addresses are compared but just to the

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