TCP/IP
Concepts
2-10
Text names make it unnecessary to remember long strings of address numbers. For example, suppose user
Bob wishes to log into a host named “alex.weasel.ctcorp.com.” The other hosts on the network have no
knowledge of which piece of hardware is associated with this name, so the ETS must translate the name into
a numeric address, such as 199.25.172.6. After the name is translated (or resolved), the connection can be
attempted.
See Set/Define Server Domain on page 12-65 for more information.If there is no local host table or name
server to provide text name translation, or if the name server(s) cannot translate the name, the host’s numeric
IP address would have to be used for the connection. Also, if the ETS has not been told which local host is
acting as the name server (see the Set/Define Server Nameserver command), it assumes that none is
available and the connect request will likely fail.
The ETS also allows the setting of a default domain name to be appended to any host name for which name
resolution is performed. In the example above, if Bob’s server had a default domain name of
weasel.ctcorp.com, then Bob could type telnet alex and the ETS would attempt to resolve
alex.weasel.ctcorp.com. This name checking occurs for all ETS commands that require text name
resolution, such as Telnet, Rlogin, and Ping. If the connect destination name ends with a period, the domain
name is not added to the hostname for resolution.
2.7.4 Local Host Table
The ETS enhances name resolution by providing a simple host table, which is a list of node names and
corresponding IP addresses. Using the Set/Define Telnet Hosts command, the server manager can add
hosts to this table. This allows the user to enter a text name described in the table in place of an IP address,
which in turn eliminates the need to remember IP addresses. The local host table is especially useful for
sites that do not have a domain name server, or in cases where a few hosts are accessed frequently.
The Show Hosts command displays all currently-known IP nodes and how they were learned: via the local
host table, a local name server, rwho broadcasts, or name resolution during a connection attempt.
If name serving is enabled, the name server will be asked for the node’s IP address only if the local host
table does not contain the information. This means that incorrect information in the local host table will be
used, even if it is contrary to what a nameserver might report. The local host table does not have to be
consistent with the name server, but consistency will avoid confusion. To maintain consistency, remove the
old hosts from the host table with the Clear/Purge Telnet Hosts command.
Host table aliases should be easy to type, remember, and recognize. They are limited to 64 characters, with
a maximum length of 16 characters per segment. For example, alex.weasel.ctcorp is legal, while
alex.communicationsserver.ctcorp is not.
The ETS can also get host names and IP addresses from node rwho broadcasts and attempt to add entries to
the host table based on this information. If the rwho addresses are cleared from the table, they will reappear
the next time the host sends out an rwho message. Addresses will also reappear if a user attempts to connect
to one of them.
2.7.5 Dynamic Host Control Protocol (DHCP)
DHCP, an extension to BOOTP, allows network administrators to lease IP addresses to network nodes as
needed. ETS servers offer two levels of DHCP support: boot and runtime.
