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 200.25.17.25 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