EXT? Std.? OpenVPN? K26? What does it mean?

Like the natural glory of double rainbows, firmware types can be not only awe-inspiring but downright mysterious as well. Even though I won't pretend for one second that I know as much as the ultra-skilled programmers on our community, I'd like to make the choices a little more clearer for our readers.

Once you've decided which firmware to choose--e.g., DD-WRT, Tomato, etc., see the guide below to help narrow down which version to download.

NOTE: This is intended to be a dynamic article, and open to comments/changes/suggestions--as such, please forgive any errors or additions that may need to be made. Please send me a PM or post a comment if you have a change or addition to suggest.

Revised 05/03/2011

Mini/Micro/Lite

Usually, any firmware build with this noted in the title has features shaved down to the bare minimum. These builds are ideal for routers with smaller space requirements, or if you have no need for features such as USB support, Samba clients, logging, etc.

Std. 

This denotes a "standard" firmware build, with a balance of features and size. These builds may or may not contain advanced features such as Samba, but usually do not offer USB support, miniDLNA support, and other advanced features.

Ext.

Generally indicates an "extended" version of firmware, which will often include features beyond the standard version.

Mega

Mega builds are generally the largest and contain most if not all available features.

USB

If you see USB in the build or filename, it means that the firmware supports USB drives being attached to the router--usually with the intent to share on your LAN or remotely. 

miniDLNA

These specialized firmware builds essentially allow you to turn your wireless router into a media server of sorts--allowing you to stream files over your local network. See this article for more info.

OpenVPN

These firmware builds include OpenVPN support. Click here for more info on OpenVPN.

K##, e.g., K26

This denotes the kernel the firmware is based on.

svn#####, e.g., svn14289

This denotes revisions of DD-WRT based firmware. Generally, the larger the number the newer the revision.

1.##.####, e.g., 1.28.7467

These are used to signify which Tomato build is used in Toastman's flavor of Tomato firmware, and possibly other versions of Tomato.

From Tomato's Website:

Tomato USB firmware builds are named according to the following specification:
tomato-<VER>-<ND|K26>[USB]-<NNNN>[MIPSR2|MIPSR1][-subver]-[EDITION].trx

• <VER> - Tomato base version (i.e. 1.27)
• <ND|K26|[Model]> - indicates the platform:
  • ND - Tomato ND ("new driver") builds based on kernel 2.4
  • K26 - Tomato builds based on kernel 2.6 (assumes ND)
  • [Model] - Special Tomato builds for certain router models
• [USB] - optional indicator of built-in USB support
• <NNNN> - build number, i.e. 9043
• <MIPSR2|MIPSR1> - architecture:
  • MIPSR2 is for MIPS32 Release 2 CPUs
  • MIPSR1 is for all other MIPS CPUs (MIPS32 Release 1)
  • Kernel 2.4-based builds are always MIPSR1, and currently are not compatible with MIPSR2 routers
• [-subver] - optional subversion number, e.g. -beta
• [EDITION] - the build edition, e.g. Ext (Extras), VPN (OpenVPN) etc.

Resources:

• Build Types via Tomatousb.org
• DD-WRT Wiki