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Configuring IP Services
1-8
The supernet address for this block is 11000000 00100000 00000, the 21 upper
bits shared by the 32-bit addresses.
A complete supernet address consists of an
address
/
mask
pair:
address
is the first 32-bit IP address in the contiguous block. In this example,
the address is 11000000 00100000 00000000 00000000 (192.32.0.0 in
decimal notation).
mask
is a 32-bit string containing a set bit for each bit position in the supernet
part of the address. The mask for the supernet address in this example is
11111111 11111111 11111000 00000000 (255.255.248.0 in dotted decimal
notation).
The complete supernet address in this example is 192.32.0.0/255.255.248.0.
Classless interdomain routing (CIDR) is an addressing scheme that employs
supernet addresses to represent multiple IP destinations. Rather than advertise a
separate route for each destination in a supernet, a router can use a supernet
address to advertise a single route — called an
aggregate
route — that represents
all of the destinations. This reduces the size of the routing tables used to store
advertised IP routes.
BGP-4 supports classless interdomain routing. OSPF supports classless routing
within a domain.
Autonomous Systems and Routing Protocols
LANs and WANs interconnected by IP routers form a group of networks called an
internet
. For administrative purposes, an internet is divided into autonomous
systems. An
autonomous system
(AS) is simply a collection of routers (called
gateways in IP terminology) and hosts. Figure 1-2 depicts a sample internet
segmented into three autonomous systems.
Routers inside an autonomous system use an
interior gateway protoco
l to
communicate network topology changes to each other. Routers in separate
autonomous systems use an
exterior gateway protocol
to communicate. The IP
router implements two dynamic interior protocols: RIP and OSPF. The IP router
implements two exterior protocols: BGP and EGP.
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