In order for routers to perform their role of routing traffic to its intended destination, they must know the identity of each network segment on the network. In the case of a LocalTalk segment, this identity is established by a network number. In the case of EtherTalk or TokenTalk segments, the identity is established by a range of contiguous network numbers known as a network range. For each network segment on an AppleTalk network, at least one router must supply the network number or range that uniquely identifies the segment.
Zone names are also linked to network segments, but do not have to be unique. This allows network segments at different locations to encompass logical groups of users. For instance, a zone named "Sales Support" could span several floors in a building with a number of network segments joined by routers.
A seed port is defined as a router port configured with a network number or network range and a zone name or zone list that identifies the network segment it is attached to. Every network segment on an extended network will have at least one router seed port active to supply its identity. A network segment may have more than one router supplying seed information, but if this is the case, each of these must absolutely agree on the information being supplied or chaos will result. Some, but not all, routers will generate alerts on startup if they detect conflicting seed information on a network segment.
A non-seed port is defined as a router port configured to obtain the network number or network range and a zone name or zone list, that identifies the network segment it is attached to from a seed router port attached to the same network segment. When a non-seed port is activated, it queries the network segment for configuration information which gets supplied in the form of a reply from a seed router on the same segment.