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IP Addressing and Subnetting 267
CIDR, Private Addressing, and NAT
When connecting to the Internet, using a registered network number or several registered
network numbers is a very straightforward and obvious convention. With registered network
numbers, no other organization connected to the Internet will have conflicting IP addresses. In
fact, this convention is part of the reason the global Internet functions well.
In the early and mid-1990s, concern arose that the available networks would be completely
assigned so that some organizations would not be capable of connecting to the Internet. This
one fact was the most compelling reason for the advent of IP Version 6 (IPv6). (The version
discussed in this book is Version 4. Version 5 was defined for experimental reasons and was
never deployed.) Version 6 calls for a much larger address structure so that the convention of all
organizations using unique groupings (networks) of IP addresses would still be reasonable--
the numbers of IPv6-style networks would reach into the trillions and beyond. That solution is
still technically viable and possibly one day will be used because IPv6 is still evolving in the
marketplace.
Three other functions of IP have been used to reduce the need for IP Version 4 (IPv4) registered
network numbers. Network Address Translation (NAT), often used in conjunction with Private
Addressing, allows organizations to use unregistered IP network numbers and still commun-
icate well with the Internet. Classless interdomain routing (CIDR) is a feature used by Internet
service providers (ISPs) to reduce the waste of IP addresses in networks so that more
organizations can be serviced by a single registered network number.
CIDR
CIDR is a convention, defined in RFC 1817 (www.ietf.org/rfc/rfc1817.txt), that calls for
aggregating multiple network numbers into a single routing entity. CIDR was actually created
to help the scalability of Internet routers--imagine a router in the Internet with a route to every
Class A, B, and C network on the planet! By aggregating the routes, fewer routes would need
to exist in the routing table. For instance, consider Figure 5-24. Class C networks 198.0.0.0
through 198.255.255.0 (they may look funny, but they are valid Class C network numbers) are
registered networks for an ISP. All other ISPs' routing tables would have a separate route to
each of the 2
16
networks without CIDR. However, as seen in Figure 5-24, now the other ISPs'
192.1.1.0,
255.255.225.224
24
5
3
30
8
128.1.0.0,
255.255.255.252
16
2
14
2
16384
Table 5-24
Examples of Number of Hosts per Subnet, and Number of Subnets (Continued)
Network and Mask
Number of
Network
Bits
Number of
Host Bits
Number of
Subnet
Bits
Number of
Hosts per
Subnet
Number of
Subnets
ch05.fm Page 267 Monday, March 20, 2000 5:06 PM