This has not yet been implemented in class Ipv4 public API; this note is in the
header file for ipv4.h:
 * TO DO:  Add API to allow access to the Ipv4 neighbor table

What I had in mind was to define some class such as Ipv4Neighbor that
corresponded to Linux struct neighbour, add some methods analogous to what we
do to add interfaces and addresses (e.g., Ipv4::GetNNeighbors(),
Ipv4::GetNeighbor (i); Ipv4::AddNeighbor (neighbor); Ipv4::DeleteNeighbor
(i);), and then add an ArpHelper class that could produce output analogous to
what you get on a Linux system when you use the "arp" command.