# REV

`REV` returns the reverse lookup domain for an IP network. For example `REV("1.2.3.0/24")` returns `3.2.1.in-addr.arpa.` and `REV("2001:db8:302::/48")` returns `2.0.3.0.8.b.d.0.1.0.0.2.ip6.arpa.`. This is used in [`D()`](https://docs.dnscontrol.org/~/revisions/T4Wre387kGCjR4Bh4mfN/language-reference/top-level-functions/d) functions to create reverse DNS lookup zones.

This is a convenience function. You could specify `D("3.2.1.in-addr.arpa", ...` if you like to do things manually but why would you risk making typos?

`REV` complies with RFC2317, "Classless in-addr.arpa delegation" for netmasks of size /25 through /31. While the RFC permits any format, we abide by the recommended format: `FIRST/MASK.C.B.A.in-addr.arpa` where `FIRST` is the first IP address of the zone, `MASK` is the netmask of the zone (25-31 inclusive), and A, B, C are the first 3 octets of the IP address. For example `172.20.18.130/27` is located in a zone named `128/27.18.20.172.in-addr.arpa`

If the address does not include a "/" then `REV` assumes /32 for IPv4 addresses and /128 for IPv6 addresses.

Note that the lower bits (the ones outside the netmask) must be zeros. They are not zeroed out automatically. Thus, `REV("1.2.3.4/24")` is an error. This is done to catch typos.

{% code title="dnsconfig.js" %}

```javascript
D(REV("1.2.3.0/24"), REGISTRAR, DnsProvider(BIND),
  PTR("1", "foo.example.com."),
  PTR("2", "bar.example.com."),
  PTR("3", "baz.example.com."),
  // These take advantage of DNSControl's ability to generate the right name:
  PTR("1.2.3.10", "ten.example.com."),
);

D(REV("2001:db8:302::/48"), REGISTRAR, DnsProvider(BIND),
  PTR("1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0", "foo.example.com."),  // 2001:db8:302::1
  // These take advantage of DNSControl's ability to generate the right name:
  PTR("2001:db8:302::2", "two.example.com."),                          // 2.0.0...
  PTR("2001:db8:302::3", "three.example.com."),                        // 3.0.0...
);
```

{% endcode %}

In the future we plan on adding a flag to [`A()`](https://docs.dnscontrol.org/~/revisions/T4Wre387kGCjR4Bh4mfN/language-reference/domain-modifiers/a)which will insert the correct PTR() record in the appropriate `D(REV())` domain (i.e. `.arpa` domain) has been defined.