January 2002
Beginner
238 pages
6h 20m
English
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Classful Versus Classless Route Lookup
To route a packet, all routers must
extract the destination IP address in the packet header. Older (or
“classful”) routers take this
address and compute its major Class A, B, or C network number (for
example, the address 172.16.1.1 belongs to the
major network 172.16.0.0). This major network
number is matched in the routing table. If there is no matching major
network number (and there is no default route in the routing table),
the packet is dropped. If there is a match against the major network
number, the router proceeds to match the subnet field. If there is no
matching subnet field in the routing table, the packet is dropped. If
there is a matching subnet field, the packet is routed as specified
in the route entry. This “classful”
routing behavior is described in more detail in Chapter 3.
Classless route lookups also refer to the destination IP address in the packet header. However, classless route lookups do not compute the major Class A, B, or C network number for the destination IP address. Instead, classless routing protocols use a rule called longest prefix match . By this rule, the destination IP address from the packet header is matched bit-by-bit against every destination IP address in the routing table. The route entry that has the longest bitwise match with the destination IP address is chosen for routing the packet.
To turn on classless route lookups, enter the following command in global configuration mode:
ip classless
To turn ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.