IP Terminology

IP Terminology
In this chapter, we’ll introduce you to a number of terms that are fundamental to an understanding
of TCP/IP. We’ll start by defining a few terms that are the most important:
Bit
Either a 1 (on) or a 0 (off).
Byte
Traditionally, eight bits, but if parity is used, user information will only be able to occupy
the high order seven bits. The use of the word
byte
in certain literature has also degraded the
meaning by referring to the most elemental number of bits as a byte. For example, the technical
documentation of a 64-bit processor may refer to a 64-bit byte. For the rest of this chapter,
always assume that a byte is eight bits.
Octet
Always eight bits.
Prefix length
The
prefix
is the part of the IP address that represents either the network
address, when the default subnet mask is used, or the network address and subnet number combined.
In other words, the prefix is everything in the IP address except the host ID. The
prefix
length
is the number of bits that the prefix extends from the highest order bit to the end of the
prefix, often presented in CIDR notation (a slash followed by the number of mask bits that are
set to 1). Subtracting the prefix length from 32 yields the length of the host ID.
A more advanced use of the term
prefix
is to describe the number of bits common to all networks
in an administrative superset (supernet) of networks or subnets of some standard prefix
length or set of prefix lengths in order to collectively refer to all of the smaller networks with
one entry. In this sense, a prefix is neither the network address nor the combination of the network
address and subnet number, but rather the number of bits that a larger group of such
networks and subnets have in common. The context of the discussion should reveal the exact
intent of the use of the term prefix.
Network address
The numeric designation used by IP devices to determine if packets should
be routed to a remote network or if they should remain on the immediate link, for example,
172.16.0.0/16 and 10.0.0.0/8. The network address identifies the segment that all hosts with
the same network address share. Although not necessarily technically accurate, it’s common to
refer to both a default prefix and the prefix of a subnetted address as the network address.
Subnet address
Also known as the
subnet number
, the numeric portion of an IP address that
was part of the host ID, by default, but which is taken over for network identification in order
to more efficiently use address space by customizing the number of networks under our administrative
control and the number of hosts on any and all such networks.
Host address or identifier (ID)
The numeric portion of an IP address that uniquely identifies a
router or host on an IP network. It’s sometimes necessary to consider the binary representation of
the host ID in order to truly understand it, especially when its boundary does not coincide with
that of an octet. Examples of host IDs are 1.1 in 172.16.1.1/16 and 1.1.1 in 10.1.1.1/8.
Broadcast address
Used by applications and hosts to send information to all nodes on a network
and characterized by all binary 1s in the host ID portion of the IP address, for example,
172.16.255.255/16 and 10.255.255.255/8.