Information Technology Reference
In-Depth Information
23.6.4 Specifying subnet masks
If a subnet is used then a bit mask, or subnet mask, must be specified to show which part of
the address is the network part and which is the host.
The subnet mask is a 32-bit number that has 1's for bit positions specifying the network
and subnet parts and 0's for the host part. A text file called
hosts
is normally used to set up
the subnet mask. Table 23.3 shows example subnet masks.
Table 23.3
Default subnet mask for type A, B and C IP addresses
Address Type
Default mask
Class A
255.0.0.0
Class B
255.255.0.0
Class C and Class B with a subnet
255.255.255.0
To set up the default mask the following line is added to the
hosts
file.
Hosts file
255.255.255.0defaultmask
23.7 Domain name system
An IP address can be defined in the form
WWW
.
XXX
.
YYY
.
ZZZ
, where
XXX
,
YYY
,
ZZZ
and
WWW
are integer values in the range 0 to 255. On the Internet, with Class B IP addresses, it is
WWW
.
XXX
.
YYY
that normally defines the subnet and
ZZZ
that defines the host. Such names
may be difficult to remember. A better method is to use symbolic names rather than IP ad-
dresses.
Users and application programs can then use symbolic names rather than the IP ad-
dresses. The directory network service on the Internet determines the IP address of the
named destination user or application program. This has the advantage that users and appli-
cation programs can move around the Internet and are not fixed to an IP address.
An analogy relates to the public telephone service. A telephone directory contains a list
of subscribers and their associated telephone numbers. If someone looks for a telephone
number, first the user name is looked up and then the associated telephone number found.
The telephone directory listing maps a user name (symbolic name) to an actual telephone
number (the actual address).
Table 23.4 lists some Internet domain assignments for World Wide Web (WWW) serv-
ers. Note that domain assignments are not fixed and can change their corresponding IP ad-
dresses, if required. The binding between the symbolic name and its address can thus change
at any time.
