Which portion of the network layer address does a router use to forward packets?

A router uses the network portion of the network layer address to forward packets. Here's a step-by-step explanation:

1. When a packet arrives at a router, the router examines the destination IP address of the packet.

2. The router compares the network portion of this IP address with entries in its routing table. The routing table is a database that stores known networks along with the interfaces to reach them.

3. The network portion of an IP address is determined by the subnet mask. For example, if the subnet mask is 255.255.255.0, then the first three octets (in a typical IPv4 address) will be the network portion.

4. Once the router identifies which network the packet belongs to, it forwards the packet to the next hop on the path to that network.

5. If the router can't find a specific match in its routing table, it sends the packet to its default gateway.

So, in summary, a router uses the network portion of the network layer address to forward packets.