Subnet and Gateway

Reference: [Book] Network Introduction for IT Engineers

• Early networks were designed considering local networks (LANs) where all terminals existed in a single network, making the communication method very simple

  • With the development of Internet technology, small LAN networks were combined into one large network, and communication with other LANs at distant locations became important

• Communication within the same network and communication between remote networks differ in both operation methods and required network equipment

  • The equipment used for communication with remote networks is called a Gateway, and Layer 3 devices (routers and switches) can fulfill this role

1. Purpose of Subnet and Gateway

• In a Local network, ARP Broadcast can be used to learn the destination MAC address and communicate directly using that MAC address,

  • But remote network communication requires the help of network equipment because broadcast cannot be transmitted beyond the network

    • This equipment is called the Gateway,

    • And the configuration item for setting gateway information on PCs and network devices is called the Default Gateway

      • The Default Gateway is performed by a Layer 3 device,

      • And plays the role of designating appropriate paths while connecting with multiple networks

• Whether the source and destination are in the same LAN or different networks changes the communication method, so the source must verify whether the destination is within its own network range

  • The Subnet mask is used for this purpose

    • The method to find your own network is to perform an AND operation on your IP address and the Subnet mask

      • If the destination belongs to this computed local network address, it is Local communication; otherwise, it is Remote communication

  • In other words, the subnet mask is used to distinguish between communication within the same network and communication between different networks

Proxy-ARP

• Proxy ARP is literally a feature that acts as a proxy for ARP

• Remote communication requires sending ARP requests to the Default gateway and sending packets toward the default gateway to communicate

  • However, if Proxy ARP is enabled on the Default gateway, even for remote communication, ARP broadcast can be sent to the local network for communication

• A default gateway (router) with Proxy ARP enabled will respond to ARP on behalf when an ARP broadcast comes in

  • In this case, the packet is sent toward the default gateway, so it can be forwarded to the remote path

• The Proxy ARP feature is enabled by default on routers and often operates without the user's knowledge

  • While it may seem convenient to operate automatically without user configuration, it often operates even when there are configuration errors in the network or essential settings are missing, making it a failure factor that prevents easy resolution during outages

    • ex) For remote communication,

      • If a router with Proxy ARP enabled is operating, it will accept and process even wrong ARP requests, so communication may work even if the Default gateway is incorrectly entered on the PC!

2. Layer 2 Communication vs Layer 3 Communication

• Layer 2 communication is Local Network communication, and Layer 3 communication is Remote Network communication

• Communication between terminals starts from the application layer and goes through Encapsulation and Decapsulation,

  • For direct communication in a Local Network (when source and destination are in the same network), communication is possible without the help of Layer 3 network devices like routers

    • Network devices connecting terminals only check information up to Layer 2 for communication, and ARP requests are sent using broadcast directly, so this is called Layer 2 communication

  • On the other hand, when communicating with a remote Network (when source and destination are in different networks), communication is impossible without the help of Layer 3 devices like routers

    • Network devices forwarding the packet must check up to Layer 3 information, and this is called Layer 3 communication

• The communication method changes based on whether the source and destination networks are "the same" or "different", and this difference arises from the different ARP operation methods for Local and Remote communication

  • Communication between terminals in the same network occurs directly

    • ARP Broadcast is used to discover the other party's MAC address,

    • And as soon as the MAC address is discovered, packets are encapsulated and communication begins

  • When communication with an external network is needed,

    • The terminal determines that the destination is at a location it cannot reach directly and sends the ARP request to the Default gateway's IP address

    • After receiving the ARP response from the Gateway, the terminal writes the Default gateway's MAC address as the destination MAC address and starts communication

• In Local communication (L2 communication), the destination MAC address and destination IP address match,

  • In Remote communication (L3 communication), the destination MAC address and destination IP address differ

    • The destination IP address is the actual destination of the communication,

    • The destination MAC address is the default gateway's MAC address