Networking - The OSI Seven-Layer Model - The Network Layer

The purpose of the Network layer is to move packets from one logical network to another. Where the Data Link layer is concerned with a single physical segment, the Network layer is concerned with building a path across many segments.

In particular, the Network layer is responsible for:

• Providing logical addressing so that hosts can be uniquely identified on an internetwork,
• Selecting paths through an internetwork (routing), and
• Forwarding packets from one interface to another based on those paths.

Various network devices are normally associated with the Network layer:

• Routers (dedicated devices that route packets between networks),
• Layer 3 switches (switches that also perform routing based on Network layer addresses), and
• Firewalls and gateways that make forwarding decisions based on Network layer information.

The Network layer is responsible for several processes, each of which is responsible for a series of methods:

1. Logical Addressing
   • Hierarchical addressing
     • Network portion (identifies the logical network),
     • Host portion (identifies a particular node on that network).
   • Examples
     • IP addresses (for the TCP/IP protocol suite),
     • IPX addresses (for Novell NetWare), and
     • Other protocol specific logical addressing schemes.
2. Path Selection and Routing
   • Static routing
     • Manually configured routes that rarely change.
   • Dynamic routing
     • Distance vector protocols (for example, RIP),
     • Link state protocols (for example, OSPF), and
     • Hybrid and exterior gateway protocols.
   • Route metrics
     • Hop count,
     • Bandwidth and delay, and
     • Administrative preferences.
3. Packet Forwarding and Delivery
   • Forwarding decisions based on destination logical address,
   • Time to live or hop limit processing to prevent endless circulation of packets, and
   • Handling of unreachable destinations and delivery notifications.
4. Fragmentation and Reassembly
   • Breaking large packets into smaller pieces to cross links with smaller maximum transmission units, and
   • Reassembling the fragments at the destination Network layer.

Let us move on to layer 4, the Introduction

Copyright 1999, Marc Elliot Hall, DBA Sensation! Services