The External Infrastructure is defined as the portion of an organisation's IT network that is accessible from external networks; such as the Internet.

In most cases the External Infrastructure is continually growing as new services are added and existing ones are expanded or physically moved to be accessible externally.

External Infrastructure Assessments have become increasingly important to providing assurance that the integrity of the Internal network is protected and sensitive services; inside perimeter defences such as border routers and Firewalls; are protected.

Specific concerns range from the hosts offering external services being susceptible to direct attack to the discovery of services that should not be present.