Gateway is also called inter-network connector and protocol converter. Gateway realizes network interconnection above the network layer. It is the most complex network interconnection device, which is only used for the interconnection of two networks with different high-level protocols. The gateway can be used for both WAN interconnection and LAN interconnection. A gateway is a computer system or device that is responsible for the conversion task.
The gateway is a translator used between two systems with different communication protocols, data formats or languages, or even completely different architectures. Unlike the bridge, which simply conveys information, the gateway repackages the received information to meet the needs of the target system.
In the TCP/IP model, a gateway is essentially an IP address from a network to other networks. (This IP address is the IP address of the device with routing function. The default gateway is also a gateway and the IP address of the device with routing function. Note that when filling in the default gateway, the IP address of the host must be in the same segment as the IP address of the default gateway.) Let’s get down to business. Let’s see how the host works with or without the gateway:
1. When there is no gateway, the host will perform ARP query on the current network and ask for the MAC address information of the destination address. If the gateway knows how to get to the destination address and the proxy ARP function is enabled, it will respond to the ARP response.
The content of the response is the destination address. The MAC address is its own, which is unknown to the PC or host router. It thinks that the MAC is the corresponding destination host. Each time it sends information to the destination address, the layer 2 encapsulates the destination MAC and sends it out.
2. When there is a gateway, the host will only ask about the MAC of the gateway. If the gateway responds, the PC or host router will send the packet, regardless of whether the gateway knows how to go to the destination address.
It can be proved that:
1. In the absence of a gateway, the proxy ARP of R1 which is turned on by default is turned off, and the ARP cache is cleared off.
Because the R1 interface has closed the proxy ARP, it will not respond to the ARP request.
2. Set the gateway as the R1 interface on R2, and send a session to the telnet of 2.2.2.2 to see if the TCP session will be sent. If it goes out, it will prove that the previous statement is correct. Whether the gateway knows the destination is reachable or not, the PC or host router will send it.
Telnet3.3.3.3 indicates that the destination is unreachable or that the gateway and host are down.
First, the MAC address of 12.1.1.1 (GW) is selected as the request for ARP sent by the router. R1 responds to this ARP request. Then a TCP session is sent out, and the protocol concludes that the remote host is unreachable.
Summary: the gateway in the host bridges the communication between two network segments (proxy ARP), and the target data layer 2 encapsulates the gateway MAC address. As for the three-layer destination network segment, whether it is reachable or not cannot be decided. ccnp encor book free download pdf
