Debian installs with DHCP IP(s) by default. Update the /etc/network/interfaces file thusly (updating the interface name and IP addressing information as necessary):

# The primary network interface
auto ens18
iface ens18 inet static
    address 192.168.1.51/24
    gateway 192.168.1.1
    dns-nameservers 192.168.1.1

Reboot the machine afterwards.