What action does a DNS server take if it does not have an entry for a requested URL?

If a DNS server does not have an entry for a requested URL, it takes the following steps:

1. The DNS server will first check its own records to see if it has the IP address for the requested URL in its cache.

2. If it does not have the record, the DNS server will then query other DNS servers. This process is known as a recursive query.

3. The DNS server will start by asking the root DNS servers for the domain. The root servers will not have the exact IP address but they will direct the query to a DNS server that is closer to the information, such as a DNS server responsible for all .com domains.

4. This process continues with each DNS server directing the query closer to the server with the actual IP address. This could involve several more steps, with the query being directed to a top-level domain (TLD) server, then to a second-level domain (SLD) server, and so on.

5. Once the DNS server finds another DNS server that knows the IP address, it will return the IP address back through the chain until it reaches the original DNS server.

6. The original DNS server will then return the IP address to the client that made the original request.

7. The DNS server will also store the IP address in its cache so if it gets another request for the same URL, it will have the IP address ready and won't need to go through the whole process again.

This entire process is done to translate the human-friendly URL into a machine-friendly IP address.