This bachelor thesis concerns the development of the web application ACLgen. ACLgen is asystem for managing and generating access control lists for firewalls. The reason behind theproject is expressed as a need for a modern take on an already existing application that is outof-date and no longer being maintained, to improve the day-to-day workflow for a networkengineer. The goal is to build the foundation for a web application that meets this need, withfurther development in mind.With ACLgen, a network engineer can generate rules and create abstract objects such ashosts and services for re-usability through a web-based user interface. Changes are saved locallyand are visually tracked in the user interface, so that the user may see new additions andchanges before committing them to the server. The system supports storing rules along withtheir related objects in a database, to persist the data. It also allows for managing multiplenetworks by creating additional repositories and making separate sets of firewall rules.The team takes advantage of modern technologies and development methods such asNext.js, Django REST framework, and Scrum for agile development. Using these technologieshas allowed the team to build expandable core systems and a user interface with solidinteraction mechanisms to prevent creating invalid rules and objects.This report focuses on exploring the possibilities and problems related to adapting andimproving a native legacy application into a web-based solution utilizing a modern softwarestack without compromising on functionality. Research and development methods wereadapted in accordance with the environment constraints and a small user base. As a result,the team has come up with solutions and suggestions on the uncovered challenges of thelegacy software adaptation.