IPAddr#addr_mask

# File tmp/rubies/ruby-3.4.1/lib/ipaddr.rb, line 755
def addr_mask(addr)
  case @family
  when Socket::AF_INET
    return addr & IN4MASK
  when Socket::AF_INET6
    return addr & IN6MASK
  else
    raise AddressFamilyError, "unsupported address family"
  end
end