Question

When a DNS request/reply is sent to/by a DNS server on another subnet, what IP destination address will be used in the IP header?

Answer 1

DNS is a normal UDP protocol: The real IP-address of the Client-Host and DNS-Server is sent (it will only change if there is a NAT-Service in between).

Example:

Host: 1.1.1.1
NAT-Router-Service: private 1.1.1.2 / public 2.2.2.2
DNS-Server: 3.3.3.3

1. Host: request-packet from 1.1.1.1 to 3.3.3.3
2. NAT-Service: request-packet from 2.2.2.2 to 3.3.3.3
3. DNS-Server: reply-packet from 3.3.3.3 to 2.2.2.2 
4. NAT-Service: reply-packet from 3.3.3.3 to 1.1.1.1

Regards Thomas