# Identifying the correct source of packets

I have a basic question regarding digital communication , especially pertaining to wireless internet .

A packet is basically a stream of bits . And a bit is detected as zero or 1 depending on whether the amplitude of a signal of certain frequency is crossing a threshold or not . Now suppose a particular host or source $$A$$ is sending this signals at a frequency $$\ f$$ to a receiver $$R$$ Then it is possible that some device , $$B$$ near the receiver can be placed to detect the frequency and regenerate a signal in that frequency and direct it towards the receiver .

In internet communication , I guess the packets will definitely be containing information about the source IP address, the IP of $$A$$ but $$B$$ can alter the bits so as to insert send its own IP address in the packets .

In such situations , how do real receivers , identify the correct source ?