Prefixes And Cidr - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

Prefixes and CIDR

A prefix describes a set of IP addresses that can be reached using the route. For
example, the prefix 10.1.1.0/24 indicates all IP addresses whose first 24 bits contain
the value 10.1.1. The term network is sometimes used instead of prefix to describe
a set of addresses. To reduce confusion, this chapter restricts network to its more
common usage, to refer to a physical structure of routers and links.
Prefixes are made possible by classless interdomain routing (CIDR). CIDR addresses
have largely replaced the concept of classful addresses (such as Class A, Class B, and
Class C) in the Internet. Classful addresses have an implicit, fixed-length mask
corresponding to the predefined class boundaries. For example, 192.56.0.0 is a Class
B address with an implicit (or natural) mask of 255.255.0.0.
CIDR uses network prefixes and explicit masks, represented by a prefix length,
enabling network prefixes of arbitrary lengths. CIDR represents the sample address
above as 192.56.0.0/16. The /16 indicates that the high-order 16 bits (the first 16
bits counting from left to right) in the address mask are all 1s.
CIDR enables you to aggregate multiple classful addresses into a single classless
advertisement, reducing the number of advertisements that must be made to provide
full access to all the addresses. Suppose an ISP has customers with the following
addresses:
Without CIDR, the ISP has to advertise a route to each address, as shown in Figure
4 on page 10.
Adj-RIBs-In store unprocessed routes learned from update messages received
by the BGP speaker.
Loc-RIB contains local routes resulting from the BGP speaker applying its local
policies to the routes contained in its Adj-RIBs-In.
Adj-RIBs-Out store routes that the BGP speaker will advertise to its peers in the
update messages it sends.
192.168.128.0
192.168.129.0
192.168.130.0
192.168.131.0
192.168.132.0
192.168.133.0
...
192.168.255.0
Chapter 1: Configuring BGP Routing
Overview
9