Host Name Resolution

A DNS server or the HOSTS file is used when you use the TCP/IP utilities, such as ping. You can find the HOSTS file in the winnt\system32\drivers\etc directory. This file is not dynamic; entries are made manually. The format of the file is the following:

IP Address Friendly Name

Host Name Resolution Using a HOSTS File

The general process that occurs when using the HOSTS file for name resolution is summarized in the following steps.

1. Computer A enters a command using the host name of Computer B.

2. The HOSTS file on Computer A (in the \Systemroot\System32\Drivers\Etc directory) is parsed. When the host name of Computer B is found, it is resolved to an IP address.

3. The Address Resolution Protocol (ARP) is then used to resolve the IP address of Computer B to its hardware address. If Computer B is on the local network, its hardware address will be obtained by using the ARP cache or by sending a local broadcast asking for a reply from Computer B with its hardware address. If Computer B is on a remote network, ARP will determine the hardware address of the default gateway for routing to Computer B.

Note

Host name resolution using a Domain Name System (DNS) server is similar to the preceding steps. Instead of parsing the HOSTS file in Step 2, the DNS server looks up the host name of Computer B in its database and resolves it to an IP address.

The following types of problems can occur because of errors related to the HOSTS file:

• The HOSTS file or the DNS server does not contain the particular host name.
• The host name in the HOSTS file or in the command is misspelled or uses different capitalization. (Host names are case-sensitive.)
• An invalid IP address is entered for the host name in the HOSTS file.
• The HOSTS file contains multiple entries for the same host on separate lines; if so, the first entry is the one that is used.
• A mapping for a computer name-to-IP-address was mistakenly added to the HOSTS file (rather than LMHOSTS).