IPAddr#loopback?

# File tmp/rubies/ruby-3.0.5/lib/ipaddr.rb, line 263
def loopback?
  case @family
  when Socket::AF_INET
    @addr & 0xff000000 == 0x7f000000
  when Socket::AF_INET6
    @addr == 1
  else
    raise AddressFamilyError, "unsupported address family"
  end
end