In the router route selection process, the first priority is the longest prefix match, also known as prefix length. According to CompTIA Network+ (N10-009) routing concepts, routers examine the destination IP address in the packet and compare it to entries in the routing table. When multiple routes exist, the router selects the route with the most specific match, meaning the route with the longest subnet mask (largest number of matching bits). For example, a /30 route is preferred over a /24, and a /24 is preferred over a /16 if all match the destination.

Only after the longest prefix match is determined do other factors come into play. Administrative distance (AD) is used to determine which routing source is more trustworthy when the same network is learned from different routing protocols (e.g., OSPF vs. RIP). Metric is then used within the same routing protocol to determine the best path (e.g., hop count, cost, bandwidth). A default route (0.0.0.0/0) is used only when no more specific match exists.

Therefore, prefix length is always evaluated first in route selection.