ould use the link-local all nodes multicast address of ff02::1. Global unicast Global unicast addresses are what you might call "normal" addresses, and will be assigned to interfaces on your devices. Global unicast addresses must be 64 bits in length except when they start with 000, as listed in Table 37-1. Link-local unicast Link-local unicast addresses are described in RFC 2373 (section 2.5.8) as follows: Link-Local addresses are designed to be used for addressing on a single link for purposes such as automatic address configuration, neighbor discovery, or when no routers are present. Routers must not forward any packets with Link-Local source or destination ad- dresses to other links. So what does that all mean? If you've ever booted a Windows machine and it couldn't find its DHCP server, you've probably seen it use a 169.254.x.x address. This range (169.254.1.0-169.254.254.255) is a link-local address block in IPv4. You can't route this block over the Internet IPv6 link-local addresses reside in the network fe80::/10. IPv6 requires operating systems to assign a link-local address to every interface automatically. The format for this address is the routing prefix combined with the MAC address. The MAC address has had an ff:fe jammed in the middle, and there's a 02 at the beginning. Why? The ff:fe converts the MAC address into the Extended Unique Identifier 64 (EUI-64) format. The 02 is actually the result of flipping the seventh bit in the first octet of the MAC address. This bit is the locally administered bit, indicating that the os has altered the address. Here's a snippet from a Linux machine. All I did was configure an IPv4 address, yet there's an IPv6 link-local address already there. As you can see, the end of the address matches the MAC address, altered as I've described: [gad@cozy ~]$ ifconfig etho Link Ethernet HWaddr 00:02:55:b7:da:9d Chapter 37: IPv6 inet addr 192.168.1.200 Bcast:192.168.1.255 Mask :255.255.255.00 inet6 addr: fe80: :202:55ff:feb7:da9d/64 Scope:Link UP BROADCAST RUNNING MULTICAST MT