x.x.x.x.spec.dnsp.co

Echoes back the address you embedded in the name.

What it does

Put an address in front of spec.dnsp.co and that address comes back as the answer. It lets you test one specific value rather than DNS in general, which is useful when a single destination behaves oddly.

Example

$ dig @dnsp.co 240.10.11.12.spec.dnsp.co +short

240.10.11.12

Reading the answer

the address you asked for

Nothing rewrote that value in transit.

a different address

That particular address is being filtered or substituted, not DNS as a whole.

no answer

The value was dropped rather than rewritten, which some filters do with reserved ranges.

Notes

Reserved and bogon ranges are the interesting ones: many filters treat them specially.
Try the suspect address here before concluding your resolver is at fault.