I'm guessing that you're on a DHCP LAN. This means that everytime you reboot, there's a possibility that a new IP address would be assigned to your server (depends on the length of the DHCP lease). This is why the older IP address is no longer valid.

It's a better practice to use hostnames instead of IP addresses when making connections to servers as it's more likely for IP address to change or migrate rather than hostnames.