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According to RFC 1738: Uniform Resource Locators (URL): 3.1. Common Internet Scheme Syntax, must the host component contain a dot, e.g., "example.com"? I.e., may it be composed of only one domain label?

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No, the host doesn't have to contain a dot. I.e., yes, only one domain label is fine. Look at domain, subdomain and label ( http://tools.ietf.org/html/rfc1034#section-3.5 ):

<domain> ::= <subdomain> | " "

<subdomain> ::= <label> | <subdomain> "." <label>

<label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]

<ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>

<let-dig-hyp> ::= <let-dig> | "-"

<let-dig> ::= <letter> | <digit>

<letter> ::= any one of the 52 alphabetic characters A through Z in upper case and a through z in lower case

<digit> ::= any one of the ten digits 0 through 9

This document is referenced from http://tools.ietf.org/html/rfc1738#section-3.1 to describe the host naming rules.

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+1, but maybe you could edit in a link to RFC 1034 Sec 3.5 and an explanation that RFC 1738 references that for host name rules? –  Kevin Aug 18 '11 at 15:37
    
Thanks. I saw that. I just didn't understand how to read it (Does the end of the first line mean that <domain> can just be a space?). But, now I see that <subdomain> can just be one <label> and <domain> can just be one <subdomain>. So, your answer makes sense. Thanks. –  MattDiPasquale Aug 18 '11 at 17:00
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