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Unformatted text preview: arization within the European routers using additional bits beyond the 7 high-order bits
of the 32-bit mask.
CIDR also uses a technique whereby the best match is always the one with the longest
match: the one with the greatest number of one bits in the 32-bit mask. Continuing the
example from the previous paragraph, perhaps one service provider in Europe needs to use a
different entry point router than the rest of Europe. If that provider has been allocated the
block of addresses 22.214.171.124 through 126.96.36.199 (16 class C network IDs), routing table file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (15 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols entries for just those networks would have an IP address of 188.8.131.52 and a mask of
255.255.240.0 (0xfffff000). A datagram being routed to the address 184.108.40.206 would
match both this routing table entry and the one for the rest of the European class C networks.
But since the mask 255.255.240 is "longer" than the mask 254.0.0.0, the routing table entry
with the longer mask is used.
The term "classless" is because routing decisions are now made based on masking operations
of the entire 32-bit IP address. Whether the IP address is class A, B, or C makes no
The initial deployment of CIDR is proposed for new class C addresses. Making just this
change will slow down the growth of the Internet routing tables, but does nothing for all the
existing routes. This is the short-term solution. As a long-term solution, if CIDR were
applied to all IP addresses, and if existing IP addresses were reallocated (and all existing
hosts renumbered!) according to continental boundaries and service providers, [Ford,
Rekhter, and Braun 1993] claim that the current routing table consisting of 10,000 network
entries could be reduced to 200 entries. 10.9 Summary
There are two basic types of routing protocols: interior gateway protocols (IGPs), for routers
within an au...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12