In the context of a network, topology refers to the way the nodes of the network connect to each other.
Networks are classified by the way the links between various nodes are shared.
A bus topology has all nodes connected to the same link. The advantage of this approach is in its simplicity and cost. Contention on the link requires either control signals to arbitrate between nodes wishing to use the link at the same time, or a means of detecting (and correcting) the situation when multiple nodes attempt to transmit data on the link simultaneously (called a collision). The more nodes there are on the link, the more contention there is for access to it, and the slower the network seems to each individual node. For this reason, bus topologies are often broken down into a hierarchical mixed topology, with subsections of the network (or subnets) sharing a bus, but each subnet is connected to other subnets through a router/switch.
In a peer-to-peer (P2P) arrangement, each node on the bus (which may be a physical wire or a given radio frequency), can connect directly to any other node on the bus. In a "master/slave" (M/S) configuration, one node (the master) is responsible for coordinating which nodes are allowed to communicate, or may be responsible for taking the data from one node and then retransmitting it to the destination node. This resembles a star topology, except that all nodes use the same bus, although only one can be transmitting at a time. In a true star topology, the nodes can only see the central node. In a bus topology, all nodes can see the transmissions of other nodes, even if they ignore them.
Ethernet, Cable internet, and Wireless internet (WiFi, WiMax, etc.) use a bus topology that detects collisions and retransmits data afterwards. Before SATA hard disks, IDE cables allowed up to two hard disk drives (or floppy disk drives) per physical cable using M/S configuration. In the modern day IDE is usually used only for DVD drives.
There are security implications for this topology. Since all nodes on the bus can see the transmissions of all other nodes on the bus, each connection between two nodes must be protected by a security protocol. The most common form used on the internet is SSH (Secure Sockets), which encrypts the data in a way that only the two nodes can decrypt. Without a security protocol, any other node on the bus could observe and record any data being transferred. Software that watches for unencrypted data is called a sniffer and can be used to steal confidential information including, but not limited to, social security numbers, phone numbers, and credit card numbers. Any use of the internet in which confidential information is transmitted should be protected by the use of SSH, or another secure protocol. The HTTPS:// web protocol uses SSH, whereas HTTP:// is inherently insecure.
A ring topology connects nodes in a circular arrangement which requires data destined for a given node to be passed through each node between the sender and destination. The data transmission may be mono- or bi-directional. Token Ring networks use a ring topology and pass data (called tokens) between the nodes to indicate which node is free to transmit data. After data is transmitted, the token is released.
A line topology resembles a ring topology, except that the ends of the line do not connect. Like a ring topology, data has to pass through each node between the sender and destination.
A star topology has a central node which is responsible for routing all network traffic between the rest of the nodes. The rest of the nodes connect only to the central node. The advantage is that there is no contention on the links, but the central node becomes a bottleneck. The central node has a limited throughput, so if the demand is too high, the central node will no longer be able to manage the volume of traffic. Network switches act as the central node for a star topology. Network routers may act as the central node in a wired star topology while also acting as the master node on a bus topology for WiFi connections.
This kind of topology also has several uses. For example, a star network may be used to isolate untrusted clients from each other using separate lines directly from each one to the router, where port security prevents communication between them.
Each link in a mesh topology connects only two nodes, but each node has dedicated links to multiple other nodes. In a full mesh, each node has a dedicated link to every other node, thus the number of links will climb exponentially with the number of nodes. This is the most expensive topology for a given number of nodes, but also has the highest bandwidth. In partial meshes, the nodes have links only to some of the other nodes. This provides a slightly less expensive network with lower, but still significant, bandwidth. A mesh with only two or three nodes is indistinguishable from a ring.
A tree topology is a multi-level star topology. That is, the central node of one star is a terminal node of another star. This can happen to any number of levels.
A mixed topology contains more than one type of topology connected together. For instance, a bus topology could be connected to a star topology through one of the nodes.