网关实质上是一个网络通向其他网络的IP地址。
比如有网络A和网络B，
网络A的IP地址范围为“192.168.1.1~192. 168.1.254”，子网掩码为255.255.255.0；
网络B的IP地址范围为“192.168.2.1~192.168.2.254”，子网掩码为255.255.255.0。
在没有路由器的情况下，两个网络之间是不能进行TCP/IP通信的，即使是两个网络连接在同一台交换机（或集线器）上，TCP/IP协议也会根据子网掩码（255.255.255.0）判定两个网络中的主机处在不同的网络里。而要实现这两个网络之间的通信，则必须通过网关。
如果网络A中的主机发现数据包的目的主机不在本地网络中，就把数据包转发给它自己的网关，再由网关转发给网络B的网关，网络B的网关再转发给网络B的某个主机。网络B向网络A转发数据包的过程也是如此。
只有设置好网关的IP地址，TCP/IP协议才能实现不同网络之间的相互通信。那么这个IP地址是哪台机器的IP地址呢？
网关的IP地址是具有路由功能的设备的IP地址，具有路由功能的设备有路由器、启用了路由协议的服务器（实质上相当于一台路由器）、代理服务器（也相当于一台路由器）。
-----默认网关
如果搞清了什么是网关，默认网关也就好理解了。就好像一个房间可以有多扇门一样，一台主机可以有多个网关。默认网关的意思是一台主机如果找不到可用的网关，就把数据包发给默认指定的网关，由这个网关来处理数据包。现在主机使用的网关，一般指的是默认网关。
默认网关必须是电脑自己所在的网段（将IP地址和子网掩码作与运算，得到网段）中的IP地址，而不能填写其他网段中的IP地址。
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Default route which is also known as the gateway of last resort, is used in forwarding packets whose destination address does not match any route in the routing table. In IPv4 the CIDR notation for a default route is 0.0.0.0/0 and ::/0 in IPv6. Now since the both the host/network portion and the prefix length is zero a default route is the shortest possible match. In previous lessons in which we discussed basics of IP Routing we know that a router when performing a route lookup will select a route with longest possible match based on CIDR specifications, however if packet does not match any route in the routing table it will match a default route, the shortest possible route, if it exists in the routing table. A default route is very useful in network where learning all the more specific routes is not desirable such as in case of stub networks. A default is immensely useful when a router is connected to the Internet as without a default route the router must have the routing entry for all networks on the Internet which are in numbers of several hundred thousand, however with single route configured as the default the router will only need to know the destinations internal to the administrative domain and will forward IP packets for any other address towards the Internet using the default route.
The default gateway is a device such as a router that serves as the edge devices providing an access point to other networks and is used to forward IP packets which does not match any routes in the routing table. We usually encounter the concept of default gateways in our daily computer life. The LAN configuration in our windows requires us to specify the IP address, Subnet Mask and the Default Gateway to access the Internet. The default gateway IP address is the IP address of the CPE or Internet modems which provide the connectivity to the Internet, now since the Internet has several hundred thousand routes which we cannot install in our table, we simply tell our computer to forward all packets destined to the Internet to this device. Again the CPE will itself have a default route and gateway configured which will point to the ISP access device.
Figure below illustrates the concept of default routes and gateways.
Router R1 connects two internal networks of an organization and provides connectivity between them, it also connects to the Internet via an ISP CPE device to provide Internet access to the users. On R1 a default route is only needed to provide Internet connectivity which is configured as shown below.
Router(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.1
All packets not matching a specific match in the routing table will be matched using the default route and forwarded to 192.168.1.2 which is the default gateway for R1. Similarly on every computer connected to either of the organization’s LAN will have the IP address of the router configured as the default gateway.
Now at the packet level what happens is that when a device attached to the network needs to communicate with another device it will first check whether the other device is on the same network or another network by comparing the IP address of the other device with subnet mask assigned to itself. If it is on another network the it will create a packet with the source IP of itself and destination of of the other device. However the layer 2 frame will have the source of address of the device itself while the destination address will be the layer 2 address of the gateway. As the packet is routed by intermediate routers the packet will remain the same i.e no change in source and destination IP addresses while the layer 2 frame will change as it crosses networks i.e the source and destination mac addresses will be changed after every layer 3 hop.
This brings us to end of this lesson in which we discussed the concept of default routes and default gateways. Both of these concepts are very essential as network administrator you will encounter both of these in the daily job life.