The primary function of a Router in a computer network is to selectively route data packets based on IP addresses. A router is a networking device that connects multiple networks together and enables the transfer of data between them. It operates at the network layer of the OSI model and uses routing tables to determine the most efficient path for data to travel from the source network to the destination network.

Selective Routing of Data Packets
- A router receives data packets from various devices on a network. Each packet contains information such as the source and destination IP addresses.
- Using the destination IP address, the router consults its routing table to determine the next hop for the packet.
- The routing table contains information about the network topology, including the IP addresses and corresponding interfaces of other routers on the network.
- Based on this information, the router forwards the packet to the appropriate next hop router, which continues the process until the packet reaches its destination network.

Advantages of Selective Routing
- By selectively routing data packets, routers optimize network performance by choosing the shortest or most efficient path for data to travel.
- This helps to minimize network congestion and reduce latency, ensuring that data reaches its destination in a timely manner.
- Selective routing also enables network administrators to implement security measures such as access control lists (ACLs) to control the flow of data between networks.
- ACLs can be used to allow or deny specific traffic based on IP addresses, providing a level of network security.

Other Functions of a Router
While selective routing is the primary function of a router, it also performs other important tasks in a computer network, including:
- Network Address Translation (NAT): Routers can translate private IP addresses used on local networks into public IP addresses for communication over the internet.
- DHCP Server: Routers can act as a Dynamic Host Configuration Protocol (DHCP) server, assigning IP addresses to devices on the local network.
- Firewall: Routers can include firewall functionality, allowing network administrators to filter and block certain types of traffic for enhanced security.
- Wireless Connectivity: Many routers also provide wireless connectivity, allowing devices to connect to the network via Wi-Fi.

In conclusion, the primary function of a router in a computer network is to selectively route data packets based on their IP addresses. This ensures that data is efficiently and securely transferred between networks, optimizing network performance and providing connectivity to various devices.