Link Local Addresses

If your network doesn't have any infrastructure wich gives out IP addresses or handles it manual with static addresses, there are a way of still communicate between computers, and that is to use link local addresses. A bit simplified, it is a randomized address that isn't allowed to pass routers, at all. IPv4 private addresses can be routed through routers. They can pass hubs and switches though.

Your machine has no global address at all, but one IPv4 link local address and one IPv6 link local address.

IPv4 Link Local Addresses

If the two computers you want to communicate between are using IPv4 link local addresses, you can use them to communicate between them for example with ping(8). The link local address range is 169.254.0.0/16; see rfc3927. Notice that link local addresses have nothing to do with global addresses or private addresses in IPv4. Private addresses are some of these nets 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16; see rfc1918.

So if the computers have IPv4 addresses which are prefixed with 196.254 and are located on the same link, they can use those to communicat between them, with ping(8) or any other tool if you like. Notice that the address usually are the same, but can change between reboots and networks.

IPv6 Link Local Addresses

If the two computers are located on the same link and uses IPv6, you can use their link local adresses. All devices that support IPv6 must have a link local addresses. The address ranges of IPv6 are found in rfc4291 and is in net fe80::/64.

So any IPv6 address with the prefix fe80:: are a link local address and are uniq to the link. So you can use those addresses to reach other computers on the same link, but you must add the interface you uses as a suffix to the address (or tell the application which interface to use.) That suffix starts with a % character followed by the interface id.

ping6 ffe8::54c7:ef39:a57a:1802%11

Notice the %11 in the end of the address.

Global IPv6 addresses usually is in the net 2000::/3, starts with a 2 or 3, but are always (nearly) divided in '/64'-net on lan. But look up the rfc:s to learn more about this.