ARP (Address Resolution Protocol) typically does not work on a Wide Area Network (WAN) in the same way it does on a Local Area Network (LAN). This is because ARP relies on broadcasting, and broadcasts are generally confined to a single LAN. Let's explore why and how address resolution happens on a WAN.
Why ARP Doesn't Work on WAN Like LAN
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Broadcast Domain Limitation: ARP relies on broadcasting a request to all devices on the network. According to provided references, "the source device will broadcast the ARP request message to the local network." WANs, by design, do not function as a single broadcast domain. Routers, which connect different networks (including WANs), do not forward broadcasts. This prevents ARP requests from propagating across the WAN.
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Point-to-Point Links: WAN connections often utilize point-to-point links between routers. In such cases, the mapping between IP and MAC addresses is preconfigured or handled by other protocols.
Address Resolution on WAN
Instead of ARP, WANs use different mechanisms for address resolution and packet delivery:
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Static Configuration: In some cases, the mapping between IP addresses and the next-hop router's interface is statically configured on each router. This is common in smaller, simpler WAN setups.
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Routing Protocols: Routing protocols like BGP (Border Gateway Protocol) and OSPF (Open Shortest Path First) are used to exchange network reachability information between routers. This information includes IP address prefixes and the next-hop router to reach those prefixes.
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Frame Relay and ATM: Older WAN technologies like Frame Relay and ATM use virtual circuits (VCs) which are pre-configured paths between endpoints. The mapping between an IP address and the VC is established during the VC setup.
Example Scenario
Let's say you have two LANs connected through a WAN:
- LAN A: 192.168.1.0/24, connected to Router A
- LAN B: 192.168.2.0/24, connected to Router B
- WAN Link: Connects Router A and Router B
If a device on LAN A (e.g., 192.168.1.10) wants to communicate with a device on LAN B (e.g., 192.168.2.20), here's what happens:
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The device on LAN A checks its routing table and determines that traffic destined for 192.168.2.20 should be sent to Router A (its default gateway).
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The device on LAN A uses ARP within its LAN to find the MAC address of Router A's LAN interface. According to provided references, "the broadcast message is received by all the other devices in the LAN network"
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The device on LAN A encapsulates the IP packet within an Ethernet frame addressed to Router A's MAC address.
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Router A receives the packet. It consults its routing table and determines that the destination network (192.168.2.0/24) is reachable via Router B.
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Router A forwards the packet to Router B across the WAN link. The method used to achieve this depends on the WAN technology.
- Point-to-Point: If the WAN link is a point-to-point link, the packet might be directly encapsulated in a PPP (Point-to-Point Protocol) frame.
- Frame Relay/ATM: If using Frame Relay or ATM, the packet would be encapsulated according to the corresponding protocol and sent over the appropriate virtual circuit.
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Router B receives the packet, consults its routing table, and determines that the destination (192.168.2.20) is on its directly connected LAN B network.
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Router B uses ARP on LAN B to find the MAC address of the destination device (192.168.2.20). According to provided references, "The device whose IP address has matched with the destination IP address in the packet will reply and send the ARP Reply message".
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Router B encapsulates the IP packet in an Ethernet frame addressed to the MAC address of the destination device.
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The destination device (192.168.2.20) receives the packet.
In summary, ARP is used within each LAN segment to find the MAC address of the next hop (typically a router). Address resolution across the WAN itself is handled by routing protocols, static configuration, or other mechanisms specific to the WAN technology being used.
