Routable IP addresses and characters

Article:TECH106528  |  Created: 1996-01-06  |  Updated: 2009-01-12  |  Article URL
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Technical Solution



You are trying to use the underscore character ( _ ) or an international character in a host name when using TCP/IP, or you are trying to connect over the Internet, but the IP address is not being routed. The name is never resolved, and pcAnywhere times out while trying to connect to the host.


Valid characters
The underscore character ( _ ) is not allowed in IP names. Neither are international characters or extended ASCII characters such as ~, *, %, and so on.

According to the Internet Standards as set forth by the Internet Architecture Board (IAB), the only characters allowed in a TCP/IP host name are the numbers 0 to 9, the hyphen ( - ), the period ( . ), and the letters A to Z in either upper or lower case. The following is a quote from RFC 1034 (STD13) available from the Internet Assigned Numbers Authority (IANA), the Internet Information Center (InterNIC), and the Microsoft Knowledge Base:

"Note that while upper and lower case letters are allowed in domain names, no significance is attached to the case. That is, two names with the same spelling but different case are to be treated as if identical. The labels must follow the rules for ARPANET host names. They must start with a letter, end with a letter or a digit, and have as interior characters only letters, digits, and hyphen. There are also some restrictions on the length. Labels must be 63 characters or less."

TCP/IP does not distinguish between upper and lower case. If the only difference between two computers' names is upper case versus lower case, TCP/IP will not see a difference between the two.

Valid addresses
The Internet Assigned Numbers Authority (IANA) has set aside three blocks of addresses for private internet spaces as documented in RFC 1597. These addresses are not unique on the Internet; they are used with the assumption that they will be internal to a private network. Therefore, they are filtered by Internet routers. These addresses are as follows: -- -- --

If you try to connect to a pcAnywhere host over the Internet using an address within one of these blocks, the TCP/IP packets will be filtered before they reach their destination, and pcAnywhere will time out looking for a connection.

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