Compatible Systems 5.4 - Manual (Page 293)

Appendices 287
When a device on the network attempts to discover services (such as servers
or printers) using a Chooser program, an NBP (Name Binding Protocol)
lookup packet is sent to a router on the same segment, which then performs a
lookup in its tables to determine the network number(s)/range(s) for a partic-
ular zone. These tables are maintained using the ZIP (Zone Information
Protocol).
The lookup is then forwarded to the appropriate segment(s). Devices whose
services match the information in the lookup will respond to it, and the
response will be forwarded back to the original machine.
Router Autoconfiguration
An autoseeding default router configuration simplifies administration of
routers since AppleTalk can be routed right out of the box without any addi-
tional configuration.
To determine a network number to use for autoseeding, a router listens to the
network for several RTMP periods, and then examines its routing table
(which is filled in with information from RTMP packets), and picks an
unused number for each interface. Only Phase 2 extended networking is
turned on in the default configuration, with network ranges of 1.
A default zone name is created for each interface that incorporates the
router’s Ethernet address, which is guaranteed to be unique.
Bridging 101
v Note: This is a very brief introduction to the concept of bridging networks.
For more in-depth information, there are a variety of references, including
the IEEE 802.3d spanning tree specification. A good general purpose refer-
ence is Radia Perlman’s Interconnections (Addison-Wesley Publishing).
Bridging Basics
Bridges are used to limit the amount of traffic appearing on network segments
other than the destination segment. They do not provide for the logical
grouping of network devices, which makes them considerably less flexible
than routers from the standpoint of network management.
In contrast to routers, bridges operate on the “physical” network layer. While
protocols such as IP or IPX are concerned with their own addressing schemes
and routing tables (see IP 101 or IPX 101), bridging is only concerned with
physical (i.e. Ethernet) addresses, and which bridge interface they are
located on.