and Frame Relay mapping.) With that mapping information, R2
can complete the Frame Relay header and send the frame to R3.
R2 before it, R3 checks the FCS in the data link trailer, looks at
the type field to decide whether the packet inside the frame is an
IP packet, and then discards the Frame Relay header and trailer.
The routing table entry for 168.1.0.0 shows that the outgoing
interface is R3's Token Ring interface. However, there is no next
router IP address because there is no need to forward the packet to
another router. R3 simply needs to build a Token Ring header and
trailer and forward the frame that contains the original packet to
PC2. Before R3 can finish building the Token Ring header, an IP
ARP must be used to find PC2's MAC address (assuming that R3
doesn't already have that information in its IP ARP cache).
particulars of IP and IPX. One key feature of network layer addresses is that they were designed
to allow logical grouping of addresses. In other words, something about the numeric value of
an address implies a group or set of addresses, all of which are considered to be in the same
grouping. In TCP/IP, this group is called a network or a subnet. In IPX, it is called a network.
In AppleTalk, the grouping is called a cable range.
differ for some network layer protocols, but the grouping concept is identical for IP, IPX, and
AppleTalk. In each of these network layer protocols, all devices with addresses in the same
group cannot be separated from each other by a router that is configured to route that protocol,
respectively. Stated differently, all devices in the same group (subnet/network/cable range)
must be connected to the same data link; for example, all devices must be connected to the same
Ethernet.
each network layer protocol can reference the group, not each individual address. Imagine an
Ethernet with 100 Novell clients. A router needing to forward packets to any of those clients
needs only one entry in its IPX routing table. If those clients were not required to be attached
to the same data link, and if there was no way to encode the IPX network number in the IPX
address of the client, routing would not be capable of using just one entry in the table. This basic
fact is one of the key reasons that routers, using routing as defined by a network layer (Layer
3), can scale to allow tens and hundreds of thousands of devices.