[ALUG] Is there such a thing as a 'WiFi switch' to add WiFi to a LAN - if so what's it called?

I'm not even sure such a thing exists but it feels as if it ought to. I want a 'WiFi switch' that I can plug into an existing LAN such that it allows WiFi access on the *same* subnet. Thus it won't do anything clever like DHCP or DNS or anything as that will be done by existing devices on the LAN. All the 'range extenders' I can find are WiFi both in and out and also tend to create their own subnet. Anyway I need something that hangs on a hard-wired ethernet connection. Is there such a thing? -- Chris Green