Network Topology Simulator

Design network diagrams visually. Add routers, switches, and hosts, connect them, auto-assign point-to-point IP addresses, and export Cisco IOS configurations — all in the browser, no installation required.

Add device

Add devices from the toolbar — use Connect to link them

Click a device or link to see its properties

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How to use the topology simulator

Start by adding devices from the left toolbar — click Router, Switch, Host, Firewall, or Cloud to place nodes on the canvas. Drag nodes to reposition them. Double-click a node to rename it inline.

To connect two devices, click Connect in the toolbar, then click the source device followed by the target device. The simulator automatically selects the first available interface on each end. Click a link to inspect or remove it.

Once your topology is laid out, use Auto-assign IPs to allocate point-to-point /30 subnets from the 10.0.0.0/8 pool — one per link, starting from 10.0.0.0/30. Use Analyze to generate the adjacency table, detect loops, and identify disconnected devices. Use Export Cisco Config to download ready-to-paste IOS commands for every router.

Network topology types

Choosing the right topology is the first design decision in any network project. Each type offers different trade-offs in cost, redundancy, and scalability.

Star topology

All end devices connect to a central switch or hub. The star is the most common LAN topology because failures are isolated — a broken cable only disconnects one host. The central switch is a single point of failure, which is why enterprise designs stack or dual-home the core switch. In the simulator, place a switch at the center and connect multiple hosts to it to model a star.

Mesh topology

Every node connects directly to every other node (full mesh) or to multiple peers (partial mesh). Full mesh provides maximum redundancy but the number of links grows as n(n−1)/2, making it expensive at scale. Partial mesh is common in WAN designs where critical sites get redundant paths and branch offices get a single uplink. The simulator's loop detector will flag a full mesh — use Spanning Tree Protocol (STP) or routing protocols to manage redundancy.

Ring topology

Devices form a closed loop where each node connects to exactly two neighbors. Token Ring and SONET/SDH use ring architectures. A single link failure leaves the ring intact (traffic reroutes); a second failure isolates a segment. The simulator's Analyze button detects ring loops and warns about them — useful when verifying L2 designs before enabling STP.

Bus topology

All devices share a single backbone cable with terminators at each end. Bus was standard in 10BASE-2 (Thinnet) Ethernet. A break anywhere on the bus brings down the whole segment, so it has no place in modern designs. It is modeled in the simulator by using a single switch with all hosts attached — conceptually equivalent for modern Ethernet.

Hybrid topology

Real enterprise networks combine multiple topologies. A common pattern: a redundant core mesh between distribution-layer switches, star spokes extending to access-layer switches, and point-to-point WAN links to branch routers. The simulator supports all of these simultaneously — mix routers, switches, and hosts in any combination.

Point-to-point links and /30 subnets

Router-to-router links are always point-to-point: two endpoints, four addresses. A /30 subnet (255.255.255.252) provides exactly two usable host addresses — one for each router interface — wasting no address space. The auto-assign algorithm allocates contiguous /30 blocks from the parent pool: 10.0.0.0/30, 10.0.0.4/30, 10.0.0.8/30, and so on. RFC 3021 also permits /31 subnets for P2P links, saving one more address per link.

Reading the adjacency table

The adjacency table produced by Analyze lists every neighbor relationship in the topology. Each link appears twice — once from each endpoint's perspective — matching the format used by protocols like OSPF and IS-IS in their link-state databases. The table shows the local interface, the neighbor device and its interface, and the subnet assigned to that link. Use it to verify connectivity before configuring routing.

Exporting to Cisco IOS

The Export Cisco Config button generates a configuration block for each router in the topology. Each block includes hostname, one interface stanza per connected interface with its ip address and no shutdown, and a comment header. Paste the output directly into a Cisco router's privileged EXEC mode or into a GNS3/Packet Tracer startup config.