How do Routers Determine where a Packet needs to go?

Almost any time you access the Internet, a router is at work somewhere behind the scenes. Routers are networking devices that manage data and direct traffic to specific destinations. They do this by inspecting header information on packets travelling through them and determining where they need to go next.

How do routers determine where a packet needs to go? There are many ways routers determine where network traffic needs to go. There are different ways in which the functions of a router can be implemented. It depends on your use case. A software based solution is lighter weight but comes with less operational visibility. If you’re looking for something more robust, hardware based solutions may be more fitting.

If you’ve ever wondered how a router figures out where to send your data, you’ve come to the right place. Read the full article because here we are going to discuss how do routers determine the best path.

How Do Routers Determine the Best Path

The routing table is used to determine the best path. The routing table maps IP addresses to specific ports on the router. An IP address may be mapped to a port that is used for WAN or LAN connections.

The routing table may also have a default port number that means the IP address is not mapped to a specific port number. The routing table may also have a port number that is used to send packets to other routers.

The IP addresses that are not mapped to a specific port number are said to be directly managed by the router. The routing table will also have a metric to determine which is the best path. The routing table will have the current path as the best path.

The router will then try to find another path that has a better metric. It may also send a copy of the packet out of the current path. The router may do this when the new path has less bandwidth and the router needs to send a copy of the packet out of the current path to keep the new path from getting congested.

Checking the Destination IP

The first thing that a router does is to check the destination IP. The router’s job is to determine the best path for each IP packet. The router employs a routing table that maps IP addresses to the best path to reach those IP addresses.

The routing table is managed by the router’s operating system and is used to determine the best path by checking the Destination IP address.

Looking At the Network Layer Protocol

The router will now look at the layer 3 header information in the packet. The router first determines if the packet is TCP or UDP. The router then reads the source and destination TCP or UDP ports. TCP and UDP have source and destination ports.

By reading the source and destination ports, the router determines if the TCP or UDP packet is going to or coming from a specific application. The router then looks for a connection state table that matches a TCP or UDP port to a specific application.

If a connection state table is found, the router will use that connection state table to determine where the packet needs to go next. This is called routing based on application layer information.

Checking the MAC Address

The next thing that a router would do is to check the MAC address. The router would use the ARP table to determine how to reach the IP address that the packet’s destination MAC address is mapped to. The ARP table maps IP addresses to MAC addresses.

Once the router finds the IP address in the ARP table, the router would then look in the routing table to determine where the packet needs to go next.

Another scenario would be that the router may not see the IP address in the ARP table. In this scenario, the router would send a broadcast out to the local network and have the host with the MAC address in the packet’s destination MAC address reply back with the IP address that is mapped to that MAC address.

The router then would add the IP address to the ARP table and then look in the routing table to determine where the packet needs to go next.

Looking at the Local Network Info

The router will then look at the local network information. The router may have an IP address or subnet mask mapping to the local network. The router would then check to see if the source IP address is mapped to the local network.

If the source IP address is not mapped to the local network, the router would then check if the source IP address is directly managed by the router. If it is not, the router would drop the packet. If it is, the router would forward the packet.

If the source IP address is mapped to the local network, the router would also forward the packet. The router would send the packet out on the port that is mapped to the local network.

Default Gateways

If the IP address is directly managed by the router and is not a local network IP address, the router would then check if there is a default gateway. The router would add the default gateway to the routing table and use it to determine where the packet needs to go next.

The router would then send the packet out on the WAN or LAN port that is mapped to the default gateway. If the router does not have a default gateway, the router would drop the packet. If the IP address is not directly managed by the router and is a local network IP address, the router would then check if there is a default gateway.

The router would add the default gateway to the routing table and use it to determine where the packet needs to go next. The router would then send the packet out on the WAN or LAN port that is mapped to the default gateway. If the router does not have a default gateway, the router would drop the packet.

Using Dynamic Discovery Protocols

If the router does not have a routing table entry for the IP address, the router would send a discovery packet out to the LAN. The discovery packet would have the IP address of the host that sent the packet.

The discovery packet would have a special MAC address that is used to detect a MAC address reply. The router would then add the IP address to the routing table and use it to determine the best path. The router would then send the packet out on the port that is mapped to the IP address.

Summary

In order to determine the best path for a packet, routers may check the destination IP, the network layer protocol, the MAC address, and local network information. They may also use a discovery protocol to add a new IP address to the routing table. These are just a few ways in which routers determine the best path for network traffic.

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