NAT and Private IP Addressing:
Today, almost every local LAN network is using Private IP addressing (based on RFC1918) and then translating those private IPs to public IPs with NAT (network address translation). The private IP addresses always start with 192.168.x.x or 172.16-31.x.x or 10.x.x.x (those are the blocks of private IPs defined in RFC1918).
Private IPs example: 10.110.0.100
192.168.0.1
Public IPs:81.4.181.154
In computer networking, a gateway is a node (a router) on a TCP/IP network that serves as an access point to another network. A default gateway is the node on the computer network that the network software uses when an IP address does not match any other routes in the routing table. It is usually the IP address of the router to which your PC network is connected.
In home computing configurations, an ISP often provides a physical device which both connects local hardware to the Internet and serves as a gateway. Such devices include DSL routers and cable routers.
In organizational systems a gateway is a node that routes the traffic from a workstation to another network segment. The default gateway commonly connects the internal networks and the outside network (Internet). In such a situation, the gateway node could also act as a proxy server and a firewall. The gateway is also associated with both a router, which uses headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.
In other words, a default gateway provides an entry point and an exit point in a network.
