Internet structure: repeater, bridge, router, gateway

An internet (internetwork) is a set of separate networks (LANs, MANs, WANs) joined together by interconnecting devices so that any host can communicate with any other host. The global Internet is the largest example: home and office LANs connect to ISPs, ISPs connect to regional and national backbones, and these connect to international backbones.

Structure of an internet

     LAN-1 segment A        LAN-1 segment B
  H--H--H--[Repeater]--H--H--H
                |
            [Bridge]------ LAN-2 (H--H--H)
                |
            [Router R1]
                |  WAN link (leased line)
        +-------+--------+
        |   ISP / WAN    |
        |  [R2]----[R3]  |  backbone routers
        +---+--------+---+
            |        |
        [Router]  [Gateway]
            |        |
          LAN-3   Different network
        (H--H--H) (e.g. mainframe SNA,
                   X.25, email system)

Hosts (H) on LANs reach other LANs through bridges; LANs reach distant networks through routers and WAN links; networks with different protocol suites are joined by gateways.

Devices and the OSI layers they work at

 OSI layer      Device
 7 Application  +
 6 Presentation | Gateway (all layers)
 5 Session      |
 4 Transport    +
 3 Network      Router
 2 Data link    Bridge (and switch)
 1 Physical     Repeater (and hub)

1. Repeater

• Works at the physical layer.
• A signal weakens (attenuation) and is distorted as it travels. A repeater receives the weak signal, regenerates the original bit pattern and sends it on, so the LAN can be made longer.
• It connects two segments of the same LAN; it does not read addresses or filter traffic, so all frames and collisions pass through. Both segments are one collision domain.
• Example: Ethernet 10Base5 segments of 500 m are joined by up to 4 repeaters for 2500 m total. A hub is a multiport repeater.
• Note: a repeater regenerates, it does not merely amplify (an amplifier also boosts noise).

2. Bridge

• Works at the physical and data link layers.
• Connects two LANs or LAN segments (same or different MAC types, e.g. Ethernet to Token Ring with translation).
• Reads the MAC address in each frame and uses a forwarding table:
  • destination on the same side as source → filter (discard);
  • destination on the other side → forward;
  • unknown destination → flood.
• Transparent (learning) bridges learn addresses from the source field of frames; the spanning tree algorithm removes loops when several bridges exist.
• Divides the network into separate collision domains, so it reduces traffic and improves security. A switch is a multiport bridge.

3. Router

• Works at the network layer (layers 1–3).
• Connects different networks (LAN to LAN, LAN to WAN) that may use different technologies, and forwards packets using logical (IP) addresses.
• Keeps a routing table built by static entries or routing protocols (RIP – distance vector; OSPF – link state; BGP between ISPs) and chooses the best path by metrics such as hops, delay or cost.
• Does not forward broadcasts, so it separates broadcast domains; it can also fragment packets, do filtering (firewall), NAT and QoS.
• Example: the router at a campus connects the campus LAN to the ISP over fibre.

4. Gateway

• Works at all seven layers (mainly transport to application).
• Connects networks that use completely different protocol architectures, and performs protocol conversion: changing message format, addressing, codes and procedures.
• Examples: email gateway between SMTP and X.400; gateway between TCP/IP and IBM SNA; VoIP gateway between IP telephony and the PSTN; in common usage the "default gateway" is the router leading out of a LAN.
• Usually software on a dedicated computer; slower and more complex than routers.

Comparison

Together these devices let small LANs grow into campus networks and connect, through routers and gateways, to the global Internet.