Domain Names

A domain is a directory structure for network address naming. Electronic-mail addressing also uses domains. An email address has this format:

user@subdomain. ... .subdomain2.subdomain1.top-level-domain

The part of the address to the left of the @ sign is the local address. The local address may contain information about:

• Routing using another mail transport (for example, bob::vmsvax@gateway or smallberries%mill.uucp@gateway)

• An alias (for example, iggy.ignatz)

The receiving mailer is responsible for determining what the local part of the address means.

The part of the address to the right of the @ sign shows the domain address where the local address is located. A dot separates each part of the domain address. The domain can be an organization, a physical area, or a geographic region. In older forms the domain can show one or several computer systems.

Domain addresses are case insensitive. It makes no difference whether you use uppercase, lowercase, or mixed-case letters in the domain part of an address.

The order of domain information is hierarchical--the more local the address, the closer it is to the @ sign.

The larger the number of subdomains, the more detailed the information that is provided about the destination. Just as a subdirectory in a file-system hierarchy is considered to be inside the directory above, each subdomain in the mail address is considered to be inside the location to its right.

Table 1-1 shows the top-level domains.

!%@:: A Directory of Electronic Mail Addressing and Networks by Donnalyn Frey and Rick Adams (O'Reilly & Associates, Inc., 1993) contains a complete list of international top-level domain addresses; it is updated periodically.

For mail delivery, the network or name space domain name and the mail domain name sometimes do not match. The DNS domain name and the mail domain name must be identical. By default, the sendmail program strips the first component from the network domain name to form the mail domain name. For example, if a NIS+ domain name were bldg5.eng.acme.com, its mail domain name would be eng.acme.com.

Although mail domain addresses are case insensitive, the name space domain name is not. For best results use lowercase characters when setting up the mail and name space domain names.

This default rule for determining the mail domain name restricts the number of components you can have in the network domain name. Fortunately, you can define the mail domain name in the sendmail.cf file. You can set the m variable (for mail domain name) using either a D macro definition or an L macro definition. The former is a simple assignment, while the latter uses a lookup table (sendmailvars) maintained by the name service. The advantage of the lookup table is that you can change the mail domain name easily without having to edit the sendmail.cf file on each client.