15.1 FROM NETWORKING TO INTERNETWORKING

LANs, examined in the previous chapter, interconnect devices within a limited geographic domain using common physical and data-link technologies such as Ethernet and Wi-Fi. Within a LAN, switching enables efficient frame forwarding, and Virtual LANs (VLANs) provide logical segmentation of broadcast domains.

However, VLANs do not in themselves create scalable, wide-area communication structures. They partition broadcast domains within a single switched infrastructure, but all forwarding decisions remain confined to a common Layer-2 technology and a single administrative domain.

When multiple LANs must be interconnected—across a campus, between enterprise sites, or over metropolitan and wide-area links—the problem becomes one of internetworking: interconnecting distinct networks into a coherent communication system. An internetwork is therefore a network of networks, potentially composed of heterogeneous physical media, data-link technologies, addressing schemes, and administrative authorities.

The fundamental limitation of Layer-2 networking lies in its reliance on flat MAC addressing and broadcast-based discovery. As network scale increases, broadcast traffic becomes inefficient, forwarding tables grow large, and fault isolation becomes difficult. Moreover, MAC addressing provides no inherent hierarchy to support scalable global routing.

Internetworking therefore requires mechanisms beyond those provided by Layer-2 switching. Whereas a switch forwards frames based on MAC addresses within a broadcast domain, an internetwork must support communication across multiple independent Layer-2 domains. This function is provided by Layer-3 routing, which introduces logical, hierarchical addressing independent of the underlying physical medium.

In conceptual terms, multiple LANs—each internally switched—are interconnected by routers. Each LAN may use Ethernet, Wi-Fi, or another Layer-2 technology, but routers exchange IP packets between them. Routers:

Isolate broadcast domains.
Forward packets toward destinations using network-layer addresses.
Enable communication across heterogeneous networks.
Provide fault containment and policy enforcement between administrative domains.

The architectural transition from switched LANs to routed internetworks marks the shift from local forwarding to global routing. VLAN segmentation, discussed in Chapter 14, divides broadcast domains within a network. Routing, by contrast, interconnects broadcast domains into routed domains that may scale to regional or global dimensions.

This chapter examines the principles that enable such interconnection. It considers how communication extends beyond a single broadcast domain through the introduction of logical addressing, packet forwarding, and routing. We begin by examining the architectural boundaries within the protocol stack at which interconnection occurs, and then develop the mechanisms of network-layer addressing, routing algorithms, and end-to-end transport across internetworks.

The shift from local forwarding to global routing is implemented through devices operating at different layers of the protocol hierarchy. Some devices simply extend the physical medium; others partition Layer-2 domains; only network-layer devices perform true internetworking by forwarding packets between independent address spaces. Clarifying these distinctions establishes the boundary between network extension and routed interconnection.