Systems, methods, and machine readable and executable instructions are provided for replaying captured network traffic. A method for replaying captured network traffic can include replaying multiple captured network traffic files simultaneously on the same network device, the captured network traffic files including original network traffic captured from N original connections between C original clients and S original servers. During replaying, rewriting IP addresses and/or port number information of data packets comprising the original network traffic to reflect test network traffic from M test connections between X test clients and Y test servers, where at least X is different than C or Y is different than S. The method further includes modifying checksums, during replaying, to correct values corresponding to the rewritten IP addresses and port number information. N, C, S, M, X, and Y are positive integers.