IP is not a text format (like HTTP). It's a binary format where each field of the IPv4 header has an exactly defined offset and length. The source IP address is placed at offset 96 and has a length of 32 bit, the destination IP address sits right afterwards with the same length. Changing anything will result in new protocol definition, et voilà that's IPv6.
https://en.wikipedia.org/wiki/Internet_Protocol_version_4#He...