Brand and model default IP Address, user and password.

Most commercial routers comes with an IP address,  username and password by default. Talking about the user and password, it is assumed that the user must change this setting to customize the security of their device. However, unfortunately many users never change the default password of the router which represents a security risk for home or office network.
In many cases, the user may forget the password for their router, then it may work the default password that was setup in factory. In case you can not remember the password then refer to the user manual which has details about how. As a last resort you can manually reset the router considering that this action will erase your custom settings, however, this action can also restore the default password which is the main theme of this mini tutorial.
Each manufacturer uses a different IP address, username and password for their devices. Here's a short list of most common commercial devices in more detailed way. It is not accurate or exhaustive, because it does not include all models of routers, but it covers many routers on the market. If the router is not here or the default password does not work, be sure to write following query in the search engine "... router default password". Note that for the time of publication of this document may have come on the market other devices which may have other configurations.
The default IP address/password for your router should be one of the following:

Following models (and brands) use the default IP Address (192.168.1.254)

Westell DSL Modems (US ONLY)

Some Linksys Routers/Modems

3Com Routers/Modems

Billion Routers/Modems

Netopia / Cayman Gateways

Following models (and brands) uses the other specific IP Address. 

Alcatel

Alcatel SpeedTouch Home/Pro – IP Address: 10.0.0.138  - (no default password)
Alcatel SpeedTouch 510/530/570 – IP Address: 10.0.0.138  - (no default password)

Asus

Asus RT-N16 – IP Address: 192.168.1.1 - Default Password:  admin

Dell

Dell Wireless 2300 router – IP Address: 192.168.2.1

D-Link

D-Link DSL-500 – IP Address: 192.168.0.1 - Default Password: private
D-Link DSL-504 – IP Address: 192.168.0.1 - Default Password: private
D-Link DSL-604+ – IP Address: 192.168.0.1 - Default Password: private
D-Link DIR-505 – IP Address: 192.168.0.1 - Default Password: private

Netcomm

Netcomm NB1300 – IP Address:192.168.1.1
Netcomm NB1300Plus4 – IP Address:192.168.1.1
Netcomm NB3300 – IP Address:192.168.1.1
Netcomm NB6 – IP Address:192.168.1.1 - Default Username: admin -  Default Password: admin
Netcomm NB6PLUS4W – IP Address:192.168.1.1 Default Username: admin -  Default Password: admin -  Default WEP key: a1b2c3d4e5

Netgear

Netgear DG814 – IP Address:192.168.0.1
Netgear (Optus branded) IP Address:10.1.1.1

Linksys

Possible user and password combinations:
Username: admin
Password: admin

Username: Administrator
Password: admin

Username: admin
Password (empty password)

IP Address: 192.168.1.1