'System' -> 'Preferences' → 'Network connections'
(select tab) 'Wired' → 'Add'
Give a connection name and check auto-connect according to your choice
In IPv4 settings set 'method' -> 'manual'
'Address' → your 'IP' address
'Netmask' → 'subnet mask' number
'Gateway' → your 'gateway address'
fill the DNS servers field with your 'DNS'