Out-of-band management is the use of a trusted, dedicated management channel for accessing a network device. This data is transmitted separately from in-band data such as network traffic.
In systems management, out-of-band management involves the use of management interfaces (or serial ports) for managing and networking equipment. Out-of-band (OOB) is a networking term which refers to having a separate channel of communication which does not travel over the usual data stream.
Out-of-band management allows the network operator to establish trust boundaries in accessing the management function to apply it to network resources. It also can be used to ensure management connectivity (including the ability to determine the status of any network component) independent of the status of other in-band network components.
In computing, one form of out-of-band management is sometimes called lights-out management (LOM) and involves the use of a dedicated management channel for device maintenance. It allows a system administrator to monitor and manage servers and other network-attached equipment by remote control regardless of whether the machine is powered on or whether an OS is installed or functional.
By contrast, in-band management through VNC or SSH is based on in-band connectivity (the usual network channel). It typically requires software that must be installed on the remote system being managed and only works after the operating system has been booted and networking is brought up. It does not allow management of remote network components independently of the current status of other network components. A classic example of this limitation is when a sysadmin attempts to reconfigure the network on a remote machine only to find themselves locked out and unable to fix the problem without physically going to the machine.