The original number of the road between John's Cross and Battle in Sussex was the A21. In the 1950s that road was rerouted along what was the B2091 and the road left behind was given the number B244.
This number did not last long, however, and the road soon returned to Class I status as the A2100. It remains this number.
Starting on the A21 at John's Cross the road headed south along the relatively straight London Road. It crossed the London to Hastings railway line at a level crossing adjacent to the now-gone Mountfield Halt before continuing on into Battle. It ended on the A269 to the west of the town centre.