IPAddr#loopback?

# File tmp/rubies/ruby-4.1.0-preview1/lib/ipaddr.rb, line 295
def loopback?
  case @family
  when Socket::AF_INET
    (@addr & 0xff000000 == 0x7f000000) && prefix >= 8 # 127.0.0.1/8
  when Socket::AF_INET6
    @addr == 1 || # ::1
      (@addr >> 32 == 0xffff && (
        (@addr & 0xff000000 == 0x7f000000) && prefix >= 104 # ::ffff:127.0.0.0/104 (127.0.0.1/8)
      ))
  else
    raise AddressFamilyError, "unsupported address family"
  end
end