FHRPs (First Hop Redundancy Protocols) prevent the loss of outside LAN (Local Area Network) connectivity when the default gateway router fails. Hosts can't use two gateways because they're typically configured with only a single default gateway IP (Internet Protocol) — there's no built-in failover mechanism at the host level.

The core problem:

• End devices (PCs, phones, printers) are configured with one default gateway IPv4 address
• If that gateway router's interface goes down → all hosts lose connectivity to external networks
• This happens even if a perfectly functional redundant router exists on the same VLAN (Virtual Local Area Network)

Why not just configure two gateways on the host?

• Most operating systems only use one default gateway at a time
• Even if you configure two, there's no reliable mechanism for the host to detect the failure and switch
• The failover time would be unpredictable and different per OS

How FHRPs solve this:

• Multiple physical routers are configured to present the illusion of a single virtual router to the hosts on the LAN
• They share a virtual IP address and a virtual MAC (Media Access Control) address
• Hosts point their default gateway to the virtual IP — they never need to change
• The FHRP protocol handles all the failover logic transparently

Tip: From the host's perspective, there is only one router (the virtual one). The fact that multiple physical routers back it up is completely invisible.

Go deeper:

• First-hop redundancy protocol (Wikipedia) — frames the single-default-gateway redundancy problem and lists the protocol family.