On early computer networks a simple text file called a hosts file was created. This file mapped host names to IP addresses.
In the second sentence, we have the subject "this file", the verb "mapped", the direct object "host names", and the preposition phrase "to IP addresses".
In this context, mapping one thing to another means that the one thing represents the other. The file in question is most likely a list of pairs: the first host name represents the first IP address, the second host name represents the second IP address, and so on. The file establishes those relationships.
On early computer networks a simple text file called a hosts file that mapped host names to IP addresses was created.
Here, what was the second sentence is now a subordinate clause. The subject is the relative pronoun "that", and the rest of the clause remains the same. The entire clause "that mapped host names to IP addresses" modifies the noun phrase "a simple text file called a hosts file".
This version is grammatically correct, but it is awkward. By the time we read the predicate "was created", many of us will have trouble understanding what the intended subject is meant to be. The simple subject, the first instance of the word "file", is twelve words away from its predicate.
On early computer networks a simple text file called a hosts file was created that mapped host names to IP addresses.
This is the original model. Here, the simple subject of "was created" is only five words away. The relative clause still does the same job, although it's position has been delayed by the two-word predicate. Even the distance between the first "file" and the relative clause is shorter than twelve words. It's only seven.
You asked: Does "a hosts file was created that mapped host names to IP addresses" mean "a host file was created so that a host file was mapped host names to IP addresses"?
No. The passive voice of the predicate "was created" does change the active voice of "mapped". Host names were mapped. The host file did the mapping.
A better paraphrase would be "a simple text file was created such that this file created relationships from host names to IP addresses".
You asked: What grammar did these sentences (verb + that + verb pattern) apply to?
I think you meant to ask "what grammar applies to these sentences?" It's the same grammar that applies to any other relative clause, except that a verb happens to be between this relative clause and the noun phrase it modifies.
