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:

The Network Layer Layer 3 246x300 The Network Layer   Layer 3

  • 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 – 8
    Class B – 16
    Class C – 24
    Class A – 24
    Class B – 16
    Class C – 8
    IPX 80 32 or less (only
    significant digits listed)
    48 bits (MAC address)
    AppleTalk 24 16 or less (indicates one
    or 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:

    1. Enabling IPX routing: This is done in global configuration mode. You may also enable load sharing if you want.
    2. 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)

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