Reserved IP Addresses

This is where the address schemes define the difference between a Class A, a Class B, a Class C,
a Class D, and a Class E address. Class D is used for multicast addresses and Class E is reserved
for experimental uses. Recall from Chapter 1, “Routing Principles,” that a neat trick that you can
use to almost immediately determine the class of an IP address in binary form is to label the first
four bits A, B, C, and D. Wherever the first zero lies is the class of address you are dealing with.
No 0s in the first four bits signify a Class E address. Try it.
Some IP addresses are reserved for special purposes, and network administrators shouldn’t
assign them to nodes. Table 2.2 lists some of the members of this exclusive little club and explains
why they’re included in it. For a more complete list of special-use addresses, consult RFC 3330,
“Special-Use IPv4 Addresses,” and RFCs 1700 (page 3) and 3232, “Assigned Numbers.”
TABLE 2 . 2
Reserved IP Addresses
Address Function
Network address 0 with node address of
all 0s (0.0.0.0/8)
Original Unix general broadcast. Interpreted to mean
“this network or segment.” Source address only.
Network address 0 with node address of
all 0s (0.0.0.0/32)
Interpreted to mean “this host on this network.”
Source address only.
Network address 0 with specific node
address (0.
x
.
x
.
x
/8)
Interpreted to mean “specified host on this network.”
Source address only.
Entire IP address set to all 0s (0.0.0.0/0) Used by devices to generate the default route.
Network 127 Reserved for loopback tests. Designates the local
node and allows that node to send a test packet to
itself without generating network traffic. Should
never appear outside a host.
Node address of all 0s Interpreted to mean “this network.” Mainly
appears in routing tables and engineering documents
to refer to the entire network in general, but
to no particular node. Should never appear as a
source or destination address in a packet header.
Node address of all 1s A directed broadcast. Interpreted to mean “all
nodes” on the specified network; for example,
128.2.255.255/16 means “all nodes on network
128.2” (Class B address). Destination address only.
Entire IP address set to all 1s (same as
255.255.255.255)
Broadcast to all nodes on the current network;
sometimes called a limited broadcast or an “all
ones broadcast.” Destination address only, not to
appear outside local segment.
Review of IPv4 Addressing
43
Let’s now take a look at the different network address classes, which can be assigned to an
individual host.