Linksys made a bad turn here and essentially ruined a completely good product. WRT54G 5.0 and higher -> Uses VxWorks OS and reduced Flash Memory not compatible with most third-party firmware, although the 'VxWorks killer' utility allows some third-party open source firmware to be loaded on this and future versions. There are a lot of second hand routers around the market so if you have a choice which type to buy, ALWAYS buy the WRT54GL. The WRT54GL is technically a reissue of the version 4 WRT54G.

WRT54GL 1.0/1.1 -> Linksys released the WRT54GL (the best-selling router of all time) in 2005 to support third-party firmware based on Linux, after the original WRT54G line was switched from Linux to VxWorks, starting with version 5.

WRT54G 4.0 Switched to new SoC -> This was the first board where the L shaped motherboard was introduced so this is the first version where you can take advantage of my shield and insert it into the case WRT54G 2.0/2.1/2.2 -> Although all the mods are possible the big old motherboard taking up all the space in the case, if you modify this series you have to move your circuit outside of the box or create a completely new casing (srsly don't worth the effort) There are lots of different versions on the market so let's take a look which can be modified:

Dd wrt v24 sp2 optware