This topic would be insufficiently covered without mentioning some of the important numeric address types such as IPv6 addresses, IPv4 addresses, and MAC-48/EUI-48 addresses. However, those sorts of addresses are discussed further in a separate section called numeric network addresses.
- [#dnsnames]: DNS domain names
Every DNS name is part of a domain that is referenced by a single period. Public DNS names may end with either a TLD or a TLD followed by a dot. (The optional dot at the end of a TLD generally has no impact, so the address shown with the dot is considered to be equivilent to the same address printed without the following dot. An example of when that wasn't the case was when an actual TLD didn't handle the trailing dot correctly: for details, see Royal Pingdom's report of
.seTLD being offline for about an hour in October, 2009. The entire domain that uses Sweden's country code stopped working right.) DNS names are case-insensitive.) OpenBSD manual page for “hostname” from section 7 (Macros and Conventions) of the manual notes a common implementation of how to treat the trailing dot: “If the input name ends with a trailing dot, the trailing dot is removed, and the remaining name is looked up with no further processing.”
DNS names are not case sensitive. (However, addresses which include DNS names, such as URLs, may include case-sensitive portions of the address.)
- Allowed names/characters
- Some older standards
There may be some newer standards, although breaking older standards may break compatibility. Whether or not that may be tolerable, it may still at least be good to know about the older standards.
RFC 1123: “Requirements for Internet Hosts -- Application and Support”: Section 2 (“General Issues”) states that software handling host names “MUST handle host names of up to 63 characters and SHOULD handle host names of up to 255 characters.” TechNet: Namespace planning for DNS indicates that in Windows NT 4, each subdomain (seperated by dots, and called a “label” by this document) may be up to 63 characters.
The “LDH” rule would indicate that allowed characters include letters, digits, and hyphens. RFC 3696. RFC 1912: Common DNS Operational and Configuration Errors says that the portion of a DNS name before the dot before the TLD “may not be all numbers, but may have a leading digit (e.g., 3com.com).” However, “Note there are some Internet hostnames which violate this rule (
1776.com).” The first character must be either a letter or a digit, according to RFC 1123: “Requirements for Internet Hosts -- Application and Support”: Section 2 (“General Issues”), and more specifically, according to section 2.1: “Host Names and Numbers”. This was an update to an RFC 952: “DOD Internet Host Table Specification”'s “Assumptions” section that stated “The first character must be an alpha character. The last character must not be a minus sign”. RFC 952: “DOD Internet Host Table Specification” stated, “Single character names or nicknames are not allowed.” (However,
x.comdid exist as an online bank, and existed as a separate entity until it was bought out by PayPal.)
All TLDs “will be alphabetic” according to RFC 1123: “Requirements for Internet Hosts -- Application and Support”: Section 2 (“General Issues”) (specifically the very end of section 2.1: “Host Names and Numbers”.
When using the “Internationalized domain name” (“IDN”) (and related “Internationalized Domain Names in Applications” (“IDNA”), Unicode is supported via Punycode. This is described by RFC 3492: “Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)”.
More information about using DNS is available in the section about DNS.
- [#unc]: Uniform Naming Convention (“UNC”)
A computer's name in the format of “\\ComputerName” (without quotation marks), or a resource in the longer form of “\\ComputerName\ResourceName” (also without quotation marks). In at least some cases, it may also be rendered as “//ComputerName/ResourceName”. (Samba, in Unix, may prefer the rendering with forward slashes. Apparently Microsoft Windows itself tolerates that. However, so many people use backslashes, that using backslashes is probably the standard that is familiar by some computer programmers. Therefore, it is very conceivable that some computer programs may use backslashes.) There may also be a newer alternatives that are sometimes seen: MSDN on namespaces describes the meanings behind “\\?\” and “\\.\”
Commonly, this resource may be a shared directory (a.k.a. “shared folder”) (offered using CIFS/SMB (or perhaps some other form of filesystem provided over networking?)). Probably the next most famous type of resource is a shared printer. The main reason for that familiarity may be because shared folders and printers are shown by Windows Explorer. However, there can be other types of resources shared using UNC paths. SQL databases may also commonly be shared over a network, and be accessed using a UNC-style path. (That is an example of a resource that may be shared, and may be accessed using a UNC, but which won't typically show up in Windows Explorer.)
- Windows Security (e.g. Active Directory) Domain Names
With Windows NT, a concept of a security “domain” came out. Windows 2000 made some substantial changes and renamed the security implementation to “Active Directory”. However, many of the same fundamental principles remained, and so terms like “domain” continue to be used (even when using the newer implementations named Active Directory).
First of all, it makes sense to discuss what a domain is. A domain refers to a security database, and all of the devices (such as computers) that rely on trusting that security database. A central computer (or, in many cases, more than one central computer) called a “domain controller” (“DC”) has the information needed to determine whether a user's authentication credentials (such as a username and a password) are acceptable.
- Distinguished Name (“DN”)
Used by Lightweight Directory Access Protocol.
(The security model used by business-oriented releases of Microsoft Windows, which is a model called “Active Directory”, uses LDAP or at least does have some compatability with that LDAP standard. Perhaps because of this ability to work with LDAP, Distinguished Names are also used by the Microsoft Windows operating system.)