In general integer division is implemented using a form of long division, in binary. There is no base-10 arithmetic involved. It’s a relatively expensive operation which usually requires multiple clock cycles to complete, whereas dividing by a power of two (“bit shifting”) is trivial and can be done in hardware simply by routing the signals appropriately, without any logic gates.

The metric standard is to measure information in bits.

Bytes are a non-metric unit. Not a power-of-ten multiple of the metric base unit for information, the bit.

If you’re writing “1 million bytes” and not “8 million bits” then you’re not using metric.

If you aren’t using metric then the metric prefix definitions don’t apply.

There is plenty of precedent for the prefixes used in metric to refer to something other than an exact power of 1000 when not combined with a metric base unit. A microcomputer is not one one-thousandth of a computer. One thousand microscopes do not add up to one scope. Megastructures are not exactly one million times the size of ordinary structures. Etc.

Finally: This isn’t primarily about bit shifting, it’s about computers being based on binary representation and the fact that memory addresses are stored and communicated using whole numbers of bits, which naturally leads to memory sizes (for entire memory devices or smaller structures) which are powers of two. Though the fact that no one is going to do something as idiotic as introducing an expensive and completely unnecessary division by a power of ten for every memory access just so you can have 1000-byte MMU pages rather than 4096 also plays a part.

So you’re not remapping the source ports to be unique? There’s no mechanism to avoid collisions when multiple clients use the same source port? Full Cone NAT implies that you have to remember the mapping (potentially indefinitely—if you ever reassign a given external IP:port combination to a different internal IP or port after it’s been used you’re not implementing Full Cone NAT), but not that the internal and external ports need to be identical. It would generally only be used when you have a large enough pool of external IP addresses available to assign a unique external IP:port for every internal IP:port. Which usually implies a unique external IP for each internal IP, as you can’t restrict the number of unique ports used by each client. This is why most routers only implement Symmetric NAT.

(If you do have sufficient external IPs the Linux kernel can do Full Cone NAT by translating only the IP addresses and not the ports, via SNAT/DNAT prefix mapping. The part it lacks, for very practical reasons, is support for attempting to create permanent unique mappings from a larger number of unconstrained internal IP:port combinations to a smaller number of external ones.)