Whether devices are communicating within the same network, across local networks, or with the wider internet, specific tables like MAC, NAT, ARP, and Routing Tables play a vital role. These tables ensure that data packets know where to go, how to get there, and how to return.
But how exactly do they work?
In this post, we’ll break down these critical components of network communication and explain their roles in simple terms. Let’s dive in!
MAC Table:
Communication Within the Same Private Local Network – Separated by Switches
Used by: Switches.
- Purpose: Maps devices’ MAC addresses to specific switch ports.
- How it works: Switches dynamically populate their MAC address tables by examining incoming packets. When a data packet is received, the switch learns the sender's MAC address and the port it arrived on.
- Details: If the destination MAC address is not in the table, the switch floods the frame to all ports except the one it arrived on.
ARP Table:
Communication Between Different Private Local Networks – Separated by Routers
- Used by: Routers and hosts.
- Purpose: Maps private IP addresses to their corresponding MAC addresses.
- How it works: The ARP table maintains a list of MAC addresses associated with known IP addresses in the local network. If the MAC address for an IP is unknown, the device sends an ARP request to resolve it.
NAT Table:
Communication Between a Local Private Network and the Internet – Separated by a Router
- Used by: Routers.
- Purpose: Tracks active connections by mapping internal private IPs (and optionally ports using PAT) to public IPs.
- How it works: When a device on the private network communicates with an external system, the router replaces the private source IP with its public IP. The NAT table keeps track of each translation, ensuring responses are returned to the correct device.
Routing Table:
Communication Between a Local Private Network and the Internet – Separated by a Router
- Used by: Routers.
- Purpose: Determines the next-hop IP address for forwarding packets based on their destination IP.
- How it works: The router looks up the destination IP in its routing table to find the next hop. If the next hop is in a local network, the router uses the ARP table (or sends an ARP request) to resolve the MAC address.
The router updates the Ethernet frame with:
- Destination MAC: The resolved MAC address of the next hop.
- Source MAC: Its own MAC address.
- For requests destined for the internet, the router updates its NAT table with a record of the outgoing request.
These are the very simplified fundamentals of these networking tables. Knowing them helps to debug application issues & bugs.