We have explored a fundamental phenomenon of magnetic domain wall collision in ferromagnetic nanowires using a micromagnetic simulation. With a systematic variation of an applied field strength, the domain wall collision phenomenon is observed around the Walker breakdown. Collision dynamics is found to mainly depend on domain wall inner structures. In the case of antiparallel transverse walls, it is found that the domain wall structure is preserved even after the collision, while parallel transverse walls experience multiple collisions with switching transverse components of colliding domain walls. After the Walker breakdown, collision of two domain walls comprises of creation and annihilation of a vortex and an antivortex. It is revealed that the collision dynamics of domain walls with an antivortex structure becomes strikingly distinctive depending on the relative direction of two colliding antivortex cores. Collision of vortex walls is observed to be associated with a complex interaction among vortex, antivortex, and even a tiny unreversed magnetic domain.