Information Technology Reference
In-Depth Information
specifies the entire network). Rather than change all the addresses to some other basic network number,
the administrator can subdivide the network using subnetting. This is done by borrowing bits from the
host portion of the address and using them as a subnet field, as shown in Figure 7-4.
Figure7-4
Subnet Addresses
Class B address, before subnetting
Class B
1
0
Network
Host
Class B address, after subnetting
Class B
1
0
Network
Subnet
Host
If a network administrator has chosen to use 8 bits of subnetting, the third octet of a Class B IP address
provides the subnet number. For example, address 172.16.1.0 refers to network 172.16, subnet 1; address
172.16.2.0 refers to network 172.16, subnet 2; and so on. In today's world, the difference between subnet
bits and the natural mask has become blurred, and you will often see only a prefix length that specifies
the length of the entire mask (natural mask plus subnet bits). It is still important to understand the
difference between the natural network mask, which is determined by the network class, and the subnet
mask, because routers sometimes make assumptions based on the natural mask of an address. For
example, the natural mask of 10.1.1.1/24 is 8 bits because this is a class A network, even though the
subnet mask is 24 bits.
Subnet masks can be expressed in two forms: prefix length (as in /24), or dotted-decimal notation (As in
255.255.255.0). Both forms mean exactly the same thing and can easily be converted to the other, as seen
in Example 7-1.
Example7-1
Subnet Mask Expressed in Prefix Length and Dotted Decimal
255.255.255.0 = 11111111 11111111 11111111 00000000 = /24 bits (count the
ones).
On some media (such as IEEE 802 LANs), the correlation between media addresses and IP addresses is
dynamically discovered through the use of two other members of the Internet Protocol suite: the Address
Resolution Protocol (ARP) and the Reverse Address Resolution Protocol (RARP). ARP uses broadcast
messages to determine the hardware Media Access Control (MAC)-layer address corresponding to a
particular IP address. ARP is sufficiently generic to allow use of IP with virtually any type of underlying
media-access mechanism. RARP uses broadcast messages to determine the Internet address associated
with a particular hardware address. RARP is particularly important to diskless nodes, which may not
know their IP address when they boot.
Internet Routing
Routing devices in the Internet have traditionally been called gateways—an unfortunate term because
elsewhere in the industry, the term
gateway
applies to a device with somewhat different functionality.
Gateways (which we will call
routers
from this point on) within the Internet are organized hierarchically.
