what is the main purpose of a default gateway in a lan

The default gateway’s responsibility is to route traffic from one network to another. Generally, the router works as the default gateway, allowing devices on one network to communicate with other devices. Its main purpose in most homes and small offices is to direct Internet traffic from the local network to the cable or DSL modem, which connects to the Internet service provider (ISP) and vice versa.

Considering the network’s similarity, it is like a home. The rooms in a home are like a computer in the network. The main gate of the home is just like a default gateway. If you want to get into another home, you will use the main gate of your home and another home gate to enter someone’s home. It is the same as for a computer network; when you want to go outside your network, you will use the interface connected to outside networks.

A PC or computer that does not know the IP address of the default gateway is like a person in a home who does not know where the main gate is. They can talk to other people in the home or network, but there is no way out if they do not know the gateway address or there is no gateway.

The Host Default Gateway

When the host wants to communicate outside the local network, he uses the gateway. Usually, the host’s default gateway address is the router interface address, which is connected to the host’s local network. The host IP address and the router interface address should be on the same network. A host’s routing table usually includes a gateway address. The host can dynamically receive the IPv4 address of the default gateway from DHCP services, or we can configure it manually.

The figure below illustrates the PCs in the local network configured with the gateway’s IPv4 address of 192.168.10.1. If a default getaway is configured on the host, it creates a default route in the host’s routing table. A default route is a route a computer will take when contacting a remote network. IPv4 address 192.168.10.1 is the address of the router interface, which is the default gateway for the hosts. The default route results from the default gateway configuration and is placed in the host computer’s routing table.

If host1 sends a packet to https://networkustad.com, which is not in the same network as host1, host1 then sends the packet to the router. The router accepts the packet, accesses its routing table to decide the correct exit interface based on the destination address, and then forwards the packet out using the correct interface to reach networkustad.com.

As the figure illustrates, the router connects the local network and the internet provider. Each host device is configured with the right gateway address. The hosts are in the same subnet, so they don’t need a gateway to communicate with each other. It’s the only required destination  IP address and MAC address. The gateway address is not used when host1 sends a packet to host2 on the same network. Host1 forwards the packet directly to Host2 through the switch using the IP address of the PC-2.

The Switch Default Gateway

As we discussed earlier, the switch is a layer2 device, and layer2 devices do not need an IP address to work properly. But if you want to connect to the switch remotely for administration purposes over multiple networks, you will need to configure the SVI with an IPv4 address, subnet mask, and gateway address.

In other words, to remotely access the switch from another network using SSH or Telnet, the switch must have an SVI with an IPv4 address, subnet mask and default gateway address configured. If the switch can be accessed from a host within the local network, then the switch gateway address is not required. The default gateway address must be configured on each device that wants to communicate beyond the local network.

The gateway address is typically the address of a router interface connected to the switch. To configure a default gateway on a Cisco switch, use the “IP default-gateway” command in global configuration mode. Packets originating from host computers connected to the switch already have the gateway address configured on their host computer operating systems. So, the host computer does not need a default gateway configured on the switch. The IP address and default gateway information only use packets originating from the switch.

Host routing table

We can check the host routing table on the Windows operating system using any one of the following commands.

• route print
• netstat -r

The commands can display the host routing table. Both commands generate the same result. Entering the netstat -r command or the route print command displays the result, which has three sections related to the current TCP/IP network connections:

Interface List

This section lists and shows the MAC address and assigned interface number of every network-capable interface on the host, including Ethernet, WI-Fi, and Bluetooth adapters.

IPv4 Routing Table

This section displays all IPv4 routes, direct connections, local networks, and local default routes.

IPv6 Routing Table

This section displays all IPv6 routes, direct connections, local network, and local default routes.