In computing, out-of-band management (sometimes called lights-out management or LOM) involves the use of a dedicated management channel for device maintenance. It allows a system administrator to monitor and manage servers and other network equipment by remote control regardless of whether the machine is powered on, or if an operating system is installed or functional.
By contrast, in-band management like VNC or SSH is based on software that must be installed on the remote system being managed and only works after the operating system has been booted. This solution may be cheaper, but it does not allow access to BIOS settings, or the reinstallation of the operating system and cannot be used to fix problems that prevent the system from booting.
Both in-band and out-of-band management is usually done through the network connection, but an out-of-band management card can use a physically separated network connector if preferred. A remote management card usually has an at least partially independent power supply, and can power the main machine on and off through the network.
This article focuses mainly on out-of-band (OOB) management of servers, but also high-end network devices sometimes offer out-of-band management. Modular/blade systems with dedicated management modules often offer a dedicated OOB ethernet port or Lights out management port. Some 'top of rack' switches also offer this functionality.
A complete remote management system allows remote reboot, shutdown, powering on; hardware sensor monitoring (fan speed, power voltages, chassis intrusion, etc.); broadcasting of video output to remote terminals and receiving of input from remote keyboard and mouse (KVM over IP). It also can access local media like a DVD drive, or disk images, from the remote machine. If necessary, this allows one to perform remote installation of the operating system. Remote management can be used to adjust BIOS settings that may not be accessible after the operating system has already booted. Settings of hardware RAID or RAM clocking can also be adjusted as the management card needs no hard drives or main memory to operate.
As management via a serial port has traditionally been important on servers, a complete remote management system also allows one to talk with the server through this port (SOL console).
As sending monitor output through the network is bandwidth intensive, cards like MegaRAC use built-in video compression (versions of VNC are often used in implementing this). Devices like Dell DRAC also have a slot for a memory card where an administrator may keep server-related information independently from the main hard drive.
The remote system can be accessed either through an SSH command line interface, specialized client software, or through various web browser-based solutions. Client software is usually optimized to manage multiple systems easily.
There are also various scaled-down versions, up to devices that only allow remote reboot by power cycling the server. This helps if the operating system hangs but only needs reboot to recover.
Remote management can be enabled on many computers (not necessarily only servers) by adding a remote management card (while some cards only support a limited list of motherboards). Newer server motherboards often have built-in remote management and need no separate management card.
Remote CLI access 
Another, older, version of out-of-band management is a configuration where there is a separate network available which allows network administrators to get access to console ports of network equipment, even when these devices don't forward any traffic at that moment. If a location has several network devices, a terminal server will give access to different console ports for direct CLI access, and if there is only one device it is for some switches possible to connect a dial-in modem to an AUX port. The mentioned terminal-server can often be accessed via a separate WAN that doesn't use the managed switches and routers to connect to the central site and/or it has a modem connected via dial-in access via POTS or ISDN.
See also 
- Intelligent Platform Management Interface - a server out-of-band management standard protocol
- Desktop and mobile Architecture for System Hardware - an out-of-band management standard protocol
- Intel Active Management Technology - an Intel out-of-band management technology
- HP Integrated Lights-Out - a HP out-of-band management implementation
- Dell DRAC - a DELL out-of-band management implementation
- IBM Remote Supervisor Adapter or Integrated Management Module - an IBM out-of-band management implementation
- Management interface M1000e - an implementation for Dell blade-enclosure