What is the IP format returned by ServletRequest.getRemoteAddr()

Question

The Javadoc of ServletRequest.getRemoteAddr() does not mention anything about the format of returned IP addresses. This is annoying when implementing filters. What can we count on? Is there any official specification one can rely on? Thanks.

Answer 1

Let's start at ServletRequest#getRemoteAddr() javadoc:

getRemoteAddr

    java.lang.String getRemoteAddr()

Returns the Internet Protocol (IP) address of the client or last proxy that sent the request. For HTTP servlets, same as the value of the CGI variable REMOTE_ADDR.

(emphasis mine)

Okay, let's read CGI REMOTE_ADDR spec:

4.1.8. REMOTE_ADDR

The REMOTE_ADDR variable MUST be set to the network address of the client sending the request to the server.

   REMOTE_ADDR  = hostnumber
   hostnumber   = ipv4-address | ipv6-address
   ipv4-address = 1*3digit "." 1*3digit "." 1*3digit "." 1*3digit
   ipv6-address = hexpart [ ":" ipv4-address ]
   hexpart      = hexseq | ( [ hexseq ] "::" [ hexseq ] )
   hexseq       = 1*4hex *( ":" 1*4hex )

The format of an IPv6 address is described in RFC 3513 [15].

There, you've all possible formats.

Answer 2

It's an IPv4 or IPv6 address in standard textual notation.

• For IPv4, it's the familiar "dot-decimal notation", e.g. 192.168.254.1. I couldn't find an RFC that defines it. Wikipedia mentions that it's acceptable to use octal, binary or hexadecimal notation for any of the quads (and even to mix-and-match), but I've never seen anything but decimal.
• For IPv6, it's specified in RFC-2373.