--- title: "Internet structure: repeater, bridge, router, gateway" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:internet-structure-devices" source: https://www.bctengineer.com/q/semester_8/telecommunications/internet-structure-devices site: BCT Engineering --- # Internet structure: repeater, bridge, router, gateway Telecommunications · Chapter 8: Data Communication - Asked in: 2068 Bhadra (old course), 4+10 marks - Video: [Network Devices Explained | Hub, Bridge, Router, Switch](https://www.youtube.com/watch?v=eMamgWllRFY) — CertBros - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/internet-structure-devices - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question Draw a neat diagram of an internet structure and explain the role of repeater, bridge, router and gateway. ## Short answer A repeater regenerates signals at the physical layer to extend a LAN segment. A bridge connects LANs using MAC addresses at the data link layer. A router forwards packets across networks using IP addresses at the network layer. A gateway translates between completely different protocol suites at all layers. ## Answer 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 ```text 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 ```text 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 | Point | Repeater | Bridge | Router | Gateway | |---|---|---|---|---| | OSI layer | 1 | 1–2 | 1–3 | 1–7 | | Address used | None | MAC | IP (logical) | Application/protocol | | Joins | Segments of same LAN | LANs/segments | Different networks | Different protocol suites | | Filtering | No | Yes, by MAC | Yes, by IP, routing | Yes, conversion | | Collision domain | Same | Separate | Separate | Separate | | Broadcast domain | Same | Same | Separate | Separate | | Complexity | Lowest | Low | High | Highest | Together these devices let small LANs grow into campus networks and connect, through routers and gateways, to the global Internet. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a repeater do? A. Reads MAC addresses and filters frames B. Regenerates the signal at the physical layer, joining segments of the same LAN in one collision domain C. Converts between protocol suites D. Chooses the best IP path Answer: B. A repeater works at layer 1, regenerating bits; it does not filter, so collisions pass through. (Key point: Repeater) 2. How does a transparent bridge decide to filter or forward? A. Using IP addresses and routing tables B. Using a table learned from source MAC addresses; same side = filter, other side = forward, unknown = flood C. By amplifying all frames D. By converting protocols Answer: B. Bridges learn MAC addresses from the source field and use them to filter, forward or flood. (Key point: Bridge) 3. Which is true of a router? A. It works at layer 2 and forwards broadcasts B. It works at the network layer using logical addresses and separates broadcast domains C. It works at all seven layers D. It joins segments of the same LAN only Answer: B. Routers use IP addresses with routing tables (RIP, OSPF, BGP) and do not forward broadcasts. (Key point: Router) 4. When is a gateway required? A. To extend cable length B. To join networks with completely different protocol architectures, doing protocol conversion C. To split a collision domain D. To remove loops by spanning tree Answer: B. A gateway works up to the application layer and converts formats and procedures, e.g. SMTP to X.400 or VoIP to PSTN. (Key point: Gateway) ## Related in other subjects - [Repeater, hub, bridge, switch, router](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Anetwork-devices-working) — Computer Networks and Security, chapter 3