Network lab: subnets, routing tables and a packet walk

The Network lab view answers routing and addressing questions without real routers. It is a simulation of forwarding decisions: longest prefix match, administrative distance, next hops, TTL and MTU. It does not run router software, NAT or firewalls.

Open the example

Open the template Network lab: routing with a backup link, or press 8 in any project to open the Network lab view. Above the topology, choose an example lab such as Two offices with a floating backup route and press Open example.

The diagram shows PC1, the routers R1, R2 and R3, and a server. The blue line is the path of the packet; the badges A and B mark the start and the destination. Every device and link is also listed below the diagram, so you can do everything with the keyboard.

Send a packet

In Send a packet, choose From device (PC1) and type the destination in To address or device (192.168.20.10, the list suggests every address of the lab). The result updates after every change; Send packet announces it and moves to the result.

The Hop by hop table shows, for every device, the Matched route, the egress interface (Out), the Gateway, the TTL and What happens. PC1 uses its default route to R1, R1 its static route 192.168.20.0/24 to R2, and R2 delivers on its connected network. Return path shows the way back.

Take a link down

Under Links, untick Link up for R1 – R2. The diagram draws the link dashed with a cross, and the walk now goes PC1 → R1 → R3 → R2 → Server. R1's normal static route has lost its next hop, so the second route with Distance 5 — a floating backup route — takes over.

Open Routing tables, choose R1 under Device and read the Status and Why columns: one route is not usable because its next hop is unreachable, the backup is installed. Type an address into Look up an address on this device to see the decision in sentences. Tick Link up again, or press Ctrl+Z.

Loops, blackholes and MTU

Try the other examples:

• Aggregate without a null route: the packet bounces between Parent and Child — a forwarding loop. Open Child, tick Used on the discard route 192.168.64.0/19 and the packet is dropped at Child instead (RFC 4632 rule 2).
• PPPoE uplink: path MTU and fragmentation: 1500 bytes with “Don’t Fragment” set (IPv4) are dropped at the home router (MTU 1492). Untick it to see two fragments, or send to the IPv6 address to see “Packet Too Big”.
• Asymmetric return path over two parallel links: the reply comes back over the radio link; Return path says asymmetric.
• Link-state ring with equal-cost paths: routers with Run the link-state protocol compute their routes themselves; the Equal-cost alternatives column lists the second path.

Build your own

Use Add device with a name and Kind (router, host or switch). Open the device to edit its Interfaces (addresses with prefix length, e.g. 10.0.12.1/30), Static routes and the diagram position. Hosts need a default gateway: a route 0.0.0.0/0 with the router's address as Next hop. Connect two free interfaces with Add link. Problems such as duplicate addresses or links between different subnets appear above the device list.

Export topology saves the lab as JSON; Import topology… reads it back (also from a project file). The topology diagram downloads as SVG or PNG, routing tables and the packet walk as CSV.

Subnet planner and MTU panel

Subnet planner works without a topology: choose What to calculate — for example VLSM: one subnet per requirement — enter the Block and the subnets with their Hosts. The plan lists every subnet with its usable range; Free blocks shows what is left. Aggregate and Exclude take a prefix list.

MTU, fragmentation and TCP stacks encapsulations (Add encapsulation, e.g. PPPoE and WireGuard over IPv4) and shows the Inner MTU and TCP MSS. Open TCP throughput model for the bandwidth-delay product and the loss limit.

The planner, the route lookup and the MTU panel use the same engine as the tools Signalhouse Subnet Planner, Signalhouse Route Lookup and Signalhouse MTU calculator, which are also available to scripts and AI agents through the API and MCP. Everything is saved in your project.