I presume you have a router, so you need to enter the routers internal ip address as your default gateway on your computers..
example, Your internal network has 192.168.1.* as range
your router has 192.168.1.1 as internal address.
Your computer A has 192.168.1.2 and
computer B has 192.168.1.3.
Your settings in your ipconfiguration is for computer A is IP: 192.168.1.2 subnetmask: 255.255.255.0 and default gateway 192.168.1.1
Your settings in your ipconfiguration is for computer B is IP: 192.168.1.3 subnetmask: 255.255.255.0 and default gateway 192.168.1.1
Default gateway is the "gate" to where data packets are send when your computer does not know how to handle them.
example, you want
www.google.comfor name resolution, your compter cannot resolve the ip address by itself, so it sends it to your default gateway (your router)
the default gateway knows the adres of a dns server, to which the packets are send to resolve the ip address..
So check your settings in your router (check if your ip settings match, so if you use 192.168.100.* in your internal network, make sure that in your router you also use those settings.)