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BAT54-Rail/F..
Release
7.54
06/08
12.3
DNS
473
12.3DNS
The domain name service (DNS) is responsible in TCP/IP networks for asso-
ciating computer names and/or network (domains) and IP addresses. This
service is required for Internet communications, to return the correct IP ad-
dress for a request such as 'www.hirschmann.com' for example. However,
it's also useful to be able to clearly associate IP addresses to computer
names within a local network or in a LAN interconnection.
12.3.1 What does a DNS server do?
The names used in DNS server requests are made up of several parts: one
part consists of the actual name of the host or service to be addressed; an-
other part specifies the domain. Specifying the domain is optional within a lo-
cal network. These names could thus be 'www.domain.com' or
'ftp.domain.com', for example.
If there is no DNS server in the local network, all locally unknown names will
be searched for using the default route. By using a DNS server, it's possible
to immediately go to the correct remote station for all of the names with
known IP addresses. In principle, the DNS server can be a separate comput-
er in the network. However, the following reasons speak for locating the DNS
server directly in the BAT:
D BAT can automatically distribute IP addresses for the computers in the lo-
cal network when in DHCP server mode. In other words, the DHCP server
already knows the names and IP addresses of all of the computers in its
own network that were assigned IP addresses via DHCP. With the dy-
namic address assignments of a DHCP server, an external DNS server
might have difficulties in keeping the associations between the names
and IP addresses current.
D When routing Microsoft Networks via NetBIOS, the BAT also knows the
computer names and IP addresses in the other connected NetBIOS net-
works. In addition, computers with fixed IP addresses can also enter
themselves in the NetBIOS table and thus be known by their names and
addresses.
D The DNS server in the BAT can also be used as an extremely convenient
filter mechanism. Requests for domains can be prohibited throughout the
LAN, for subnetworks, or even for individual computers—simply by spec-
ifying the domain name.
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