Routing in a Classless Environment
Figure 33 illustrates the routing advertisements for Organization A,
which was discussed in the previous CIDR example. 
F I G U R E   3 3 .   R o u t i n g   A d v e r t i s e m e n t s   f o r   O r g a n i z a t i o n   A
Since all of Organization A's routes are part of the ISP #1's address
block, the routes to Organization A are implicitly aggregated by means
of ISP #1's aggregated announcement to the Internet. In other words,
the eight networks assigned to Organization A are hidden behind a sin 
gle routing advertisement. Using the longest match forwarding algo 
rithm, Internet routers will route traffic to host to ISP #1,
which will in turn route the traffic to Organization A. Now assume that
Organization A changes its network provider to a different ISP (ISP #2),
as illustrated in Figure 34. 
F I G U R E   3 4 .   O r g a n i z a t i o n   A   C h a n g e s   N e t w o r k   P ro v i d e r s   t o
I S P   # 2
To manage the size of the Internet routing tables, Organization A can
obtain a block of ISP #2's address space and renumber the address. This
would allow the eight networks assigned to Organization A to be hid 
den behind the aggregate routing advertisement of ISP #2. Unfortu 
nately, renumbering is a labor intensive task that could be very
difficult, if not impossible, for Organization A. 
U N D E R S TA N D I N G   I P   A D D R E S S I N G
3 8






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