THE NETWORK LAYER - LAYER 3
The Network layer of the OSI Model basically defines logical addressing and the ways that packets are moved from source to destination on a network. The functions of the Network layer can be broken down as follows:
· Message addressing
· Path determination between source and destination nodes on different networks
· Routing messages between networks
· Controlling congestion on the subnet
· Translating logical addresses into physical addresses
Logical Address Construction
Protocol
Total Address Length
Bits in Network Portion
Bits in Host Portion
TCP/IP
32
Class A - 8Class B - 16Class C - 24
Class A - 24Class B - 16Class C - 8
IPX
80
32 or less (onlysignificant digits listed)
48 bits (MAC address)
AppleTalk
24
16 or less (indicates oneor many in cable range)
8 bits or less(cynamically assigned)





Routing protocols support routed protocols. A routing protocol is used to pass messages between routers for maintaining and updating routing tables. Examples of routing protocols are RIP, IGRP, OSPF, EIGRP. Routed protocols are used to carry end-user traffic across the internetwork. Examples of routed protocols are IP and IPX
Routing Protocol Types
Protocol
Characteristics
Examples
Distance Vector
Uses hop count; views the network from its neighbors' perspective;frequnt updates; copies its routing table to neighbors
RIP, IPX RIP, IGRP
Link State
Shortest path; common view of network; event-triggered update;LSPs (link state packets) sent to all network routers
NLSP, OSPF, IS-IS
Hybrid
Distance vector with more accurate metrics; no periodic updates;only event-triggered
IS-IS, EIGRP





IPX is a member of the IPX/SPX proprietary suite of protocols. IPX operates at the Network layer. It is connectionless and uses datagrams.
NetWare terms to memorize:
· Service Advertisement Protocol (SAP) - the NetWare propocol used to advertise (update) the services available over the network.
· Routing Information Protocol (RIP) - A distance vector routing protocol similar to RIP in TCP/IP that uses ticks, hop counts and split horizon metrics.
· NetWare Link Services Protocol (NLSP) - A link state routing protocol that is the default routing protocol on NetWare 4.11 and higher.
· Novell Directory Service (NDS) - Novell's Directory Service protocol.
· NetWare Core Protocol (NCP) - Providing client-to-server connections and applications.
Configuring the router for use with the Novell NetWare IPX protocfol is a two-setp process:
Enabling IPX routing: This is done in global configuration mode. You may also enable load sharing if you want.
Assigning networks to interfaces: This step in the process assigns network numbers to each interface. Remember that multiple network numbers can be assigned as long as each uses a different encapsulation (frame) type. To apply an encapsulation type, you need to use a Cisco keyword and not the Novell frame type's name.
Novell IPX Frame Types and Cisco Keywords:
Interface
Novell Frame Type
Cisco Keyword
Ethernet
Ethernet_802.3
novell-ether (default)
Ethernet_802.2

sap
Ethernet_II

arpa
Ethernet_SNAP
:
snap
Token Ring
Token-Ring
sap (default)
FDDI
Fddi_snap
snap (default)