Representation
A computer network can in general be represented by a weighted directed graph with various node annotations. Commonly, other simpler representations suffice to study the problem at hand: Eg: undirected weighted graphs.
Themes
Problems
Protocol design
Network engineers design and implement protocols/ software to provide a certain service.
Analysis
Analysis is often done by modeling network rate mathematically, and then studying the properties implied by these models either theoretically or in simulation. Or, analysis of properties may be done by observing a system deployed in practice.
Properties of common interest are performance (eg: throughput) and security.
Distributed computing connection
The study of networks is essentially about building infrastructure for enabling distributed computing which may not be specified fully.