Windows 7 / Networking

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How IPv6 Routing Works

The header of an IPv6 packet contains both the source address of the sending host and the destination address of the receiving host. When an IPv6 packet arrives at a host, the host uses its local IPv6 routing table to determine whether to accept the packet or forward it to another host or network.

Each IPv6 node (host or router) has its own IPv6 routing table. A routing table is a collection of routes that store information about IPv6 network prefixes and how they can be reached, either directly or indirectly. On IPv6 hosts, such as computers running Windows 7, Windows Vista, or Windows Server 2008, the IPv6 routing table is generated automatically when IPv6 initializes on the system. Local administrators can use the netsh interface ipv6 commands to manage these tables by viewing them and by manually adding or removing routes.

The use of this command is discussed further later in this section.

When an IPv6 packet arrives at a physical or logical network interface on an IPv6 host, such as a multihomed computer running Windows Server 2008, the host uses the following process to determine how to forward the packet to its intended destination:

1. The host checks its destination cache to see whether there is an entry that matches the destination address in the packet header. If such an entry is found, the host forwards the packet directly to the address specified in the destination cache entry and the routing process ends.
2. If the destination cache does not contain an entry that matches the destination address in the packet header, the host uses its local routing table to determine how to forward the packet. Using the routing table, the host determines the following:
   • Next-hop address If the destination address is on the local link, the next-hop address is simply the destination address in the packet header. If the destination address is on a remote link, the next-hop address is the address of a router connected to the local link.
   • Next-hop interface This is the physical or logical network interface on the host that should be used to forward the packet to the next-hop address.
3. The host then forwards the packet to the next-hop address using the next-hop interface. The host also updates its destination cache with this information so that subsequent packets sent to the same destination address can be forwarded using the destination cache entry instead of using its local routing table.