A home gateway is the box on the wall where the internet provider's fibre ends inside a home. It is usually two devices in one. The first is the ONT, the optical network terminal, which turns the light signals in the fibre into electrical network data. The second is the Wi-Fi router, which shares that connection with every device in the home. Indian customers often call it the "fibre router" or the "modem".
The fibre that reaches the gateway is not the home's alone. It is one branch of a passive optical network (PON). At the provider's exchange, one port sends light down a fibre that is split, without any power, among 32 or 64 homes. The common version in India, GPON, carries about 2.5 gigabits per second (Gbps) towards the homes on each such fibre, shared among them. Each gateway keeps only the data addressed to it. Newer versions, such as XGS-PON, carry 10 Gbps.
Here is a typical case. A family in Coimbatore orders a 300 Mbps fibre plan. A technician runs a thin fibre cable from the junction box in the building's corridor to the flat, and screws the gateway to the wall. When it is switched on, the gateway reports its serial number to the provider's equipment at the exchange. The provider's system recognises the number and links it to the family's account. It then sends the gateway its settings: the speed of the plan, the Wi-Fi name and the Wi-Fi password. Within about ten minutes, the lights on the box turn green, and the family's phones join the Wi-Fi.
The router half then does several jobs. It gives each device in the home its own local address. It shares the home's single public internet address among all of them. It blocks connections that start from the internet, unless a device in the home asked for them; this is a basic firewall. Many gateways also have a socket for a landline phone that works over the internet, and they carry the provider's TV channels.
In India, the provider owns and manages most gateways, so it can change their settings from its office. The software that does this is managed home Wi-Fi. If the Wi-Fi from one gateway does not reach every room, the family can add Wi-Fi routers and mesh systems.
The same kind of box serves homes with other kinds of line. With fixed wireless access (FWA), the box receives a 5G radio signal instead of fibre. With cable broadband, it receives the cable TV line. The router half is the same.
Every device in the home depends on the gateway, so it is also a target for attackers. Its own firmware must receive regular security fixes from the provider.