From Network Security Wiki
  • In the case of a large web server, having hundreds of PTR records can cause the DNS packets to be much larger than normal, which can cause responses to be truncated if they exceed the DNS 512 byte UDP message limit.
  • Why DHCP uses 2 Ports(67 & 68): Otherwise any other unintended PC upon receiving this packet will reply with an ICMP Port Unreachable Message. So to avoid this useless traffic.