Network change verification means testing a planned change to a network on a software copy of that network, before the change is made on the real one. The copy is a digital twin, built by network digital twin software from the settings of every device. The practice is also called pre-change verification, or network verification.
Why test a change on a copy? Many network outages are caused not by broken equipment, but by changes that people make. An engineer adds a firewall rule, edits a route or replaces a switch, and something else stops working that nobody expected. In a large network, the person making the change cannot see all of its effects. A test on the copy shows them first.
The work has three steps. First, the engineer writes the planned change, for example the new lines for a firewall's list of rules. Second, the software applies the change to the copy only, and recalculates every path in the network. Third, it compares the paths before and after the change, and reports every connection that would newly open or close. Teams also keep a list of checks that must always pass, called intents. Examples are "the internet must never reach the HR database" and "every branch must reach the core banking servers". Every change is checked against the full list.
Imagine a port operator in Visakhapatnam. Its network team must let a new customs system, on 12 servers in one subnet, reach the port's cargo database. A subnet is a block of network addresses, such as 10.20.8.0/24, which holds 256 addresses. The engineer writes a rule that allows the subnet to reach the database servers. The software tests the rule on the copy. It reports the expected new path, to the cargo database. But it also reports a second one: the customs subnet can now reach the HR system's database too. The rule named a group of database servers, and the HR database was in that group. One of the team's intents fails: "only HR servers may reach the HR database". The engineer narrows the rule to the single address of the cargo database. The second test passes all 140 intents, and the change is made on the real firewall that night.
The idea comes from software development. Programmers test code before releasing it. Network change verification gives network engineers the same habit. Once a change has passed, it is often pushed to the devices by configuration management tools, which apply the same settings to many devices at once.
How is it different from breach and attack simulation (BAS)? BAS sends harmless test attacks through the real network, to see which defences react. Change verification sends nothing through the network. It calculates the result on the copy. Both are ways of finding a problem in a live system before it causes harm.