What should an administrator check FIRST if a new host cannot reach other subnets?

When a new host cannot reach other subnets, the first thing an administrator should check is the default gateway. The default gateway is crucial for routing traffic from one subnet to another. If the default gateway is not configured correctly, or if the host is unable to communicate with the default gateway, it will not be able to reach any external networks or subnets.

The default gateway acts as an intermediary device, typically a router, that forwards packets destined for other networks. If the host has the wrong default gateway set, or none at all, it will only be able to communicate with devices on its own local subnet and will be unable to access anything beyond that.

While checking the netmask is important for determining if the host is correctly configured within its own subnet, and DNS records are key for name resolution and reaching resources by domain names, they are not the primary concern when dealing with inter-subnet connectivity issues. Ensuring that the default gateway is functioning properly and correctly configured is the first step in troubleshooting connectivity across different subnets.