Netscape Navigator has an Application Extension Block that tells Navigator to loop the entire GIF file. The Netscape block must appear immediately after the global color table of the logical screen descriptor. Only Navigator 2.0 Beta4 or better willl recognize this Extension block. The block is 19 bytes long composed off: (note: hexadecimal equivalent supplied for programmers)
byte 1 : 33 (hex 0x21) GIF Extension code byte 2 : 255 (hex 0xFF) Application Extension Label byte 3 : 11 (hex 0x0B) Length of Application Block (eleven bytes of data to follow) bytes 4 to 11 : "NETSCAPE" bytes 12 to 14 : "2.0" byte 15 : 3 (hex 0x03) Length of Data Sub-Block (three bytes of data to follow) byte 16 : 1 (hex 0x01) bytes 17 to 18 : 0 to 65535, an unsigned integer in lo-hi byte format. This indicate the number of iterations the loop should be executed. byte 19 : 0 (hex 0x00) a Data Sub-Block Terminator.
As of Beta5, the iteration count is ignored and the loop is infinite (an iteration count of zero indicates infinite). I strongly suggest you code the count accurate, so that when iterations begin working your GIF will not need to be modified. Technically that is all that needs to be done with this block.
Source: © 1996, 1997 Royal E. Frazier Jr. Updated: March 1997