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[Chapter 16] 16.2 IPv4: The Internet Protocol Version 4
A Class E address is of the form N.M.O.P, where the most significant four bits of N are 1111. These addresses
are currently reserved for experimental use.
16.2.1.3 CIDR addresses
In recent years, a new form of address assignment has been developed. This assignment is the CIDR, or Classless
InterDomain Routing, method. As the name implies, there are no "classes" of addresses as in the classical scheme.
Instead, networks are defined as being the most significant k bits of each address, with the remaining 32-k bits being
used for the host part of the address. Thus, a service provider could be given a range of addresses whereby the first 12
bits of the address are fixed at a particular value (the network address), and the remaining 20 bits represent the host
portion of the address. This method allows the service provider to allocate up to 220 distinct addresses to customers.
In reality, the host portion of an address is further divided into subnets. This subdivision is done by fixing the first
jbits of the host portion of the address to some set value, and using the remaining bits for host addresses. And those
can be further divided into subnets, and so on. A CIDR-format address is of the form k.j.l.(m...n), where each of the
fields is of variable length. Thus, the fictional service-provider network address described above could be subdivided
into 1024 subnets, one for each customer. Each customer would have 210 bits of host address, which they could
further subdivide into local subnets.
The CIDR scheme is compatible with the classical address format, with Class A addresses using an 8-bit network
field, Class B networks using a 16-bit network address, and so on. CIDR is being adopted as this book goes to press.
Combined with new developments in IP address rewriting, there is the potential to spread out the useful life of IPv4
for many years to come.
16.2.2 Routing
Despite the complexity of the Internet and addressing, computers can easily send each other messages across the
global network. To send a packet, most computers simply set the packet's destination address and then send the packet
to a computer on their local network called a gateway. If the gateway makes a determination of where to send the
packet next, the gateway is a router. The router takes care of sending the packet to its final destination by forwarding
the packet on to a directly connected gateway that is one step closer to the destination host.
Many organizations configure their internal networks as a large tree. At the root of the tree is the organization's
connection to the Internet. When a gateway receives a packet, it decides whether to send it to one of its own
subnetworks, or to direct it towards the root.
Out on the Internet, major IP providers such as AT&T, BBN Planet, MCI, and Sprint have far more complicated
networks with sophisticated routing algorithms. Many of these providers have redundant networks, so that if one link
malfunctions other links can take over.
Nevertheless, from the point of view of any computer on the Internet, routing is transparent, regardless of whether
packets are being sent across the room or across the world. The only information that you need to know to make a
connection to another computer on the Internet is the computer's 32-bit IP address - you do not need to know the route
to the host, or on what type of network the host resides. You do not even need to know if the host is connected by a
high-speed local area network, or if it is at the other end of a modem-based SLIP connection. All you need to know is
its address, and your packets are on their way.
Of course, if you are the site administrator and you are configuring the routing on your system, you do need to be
concerned with a little more than the IP number of a destination machine. You must know at least the addresses of
gateways out of your network so you can configure your routing tables. We'll assume you know how to do that,[7] but
we will point out that if your routes are fairly stable and simple, you would be safer by statically setting the routes
rather than allowing them to be set dynamically with a mechanism such as the routed daemon.
[7] If not, you should consult your vendor manual, or one of the references in Appendix D, Paper
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