How do captive portals redirect ip addresses?

So I've been looking into captive portals recently, and I was wondering how they redirect IP addresses. I know that they redirect domain names with DNS spoofing, but as far as I know there's no way to do that with IPs. I'm probably wrong, but in that case I'd like to know what the DNS record to do it is, or how to do it in general.

Thanks

A captive portal in general uses:

• HTTP redirect (302)
• Redirect via DNS; this is called hijacking, not spoofing
• ICMP redirect

There are also other ways, but these are the most common.

What you are probably referring to is the ICMP redirect.