Mapping a Switched Network with LanTopoLog: Step-by-Step

This guide is for the admin who has inherited a switched LAN with no up-to-date diagram and needs to answer one question fast: which switch port is this device plugged into? The short answer: give LanTopoLog read-only SNMP access to your managed switches, run its three discovery steps, fix the few links it cannot resolve, and then let it watch the network. Topology discovery works without limits in the unregistered version, so you can map the whole network before deciding whether to register.

LanTopoLog and the tools around it

A scanner tells you what is alive on a subnet; LanTopoLog tells you where each device physically connects.

For the wider inventory picture (hardware, software, help desk), see our IT asset management and help desk tools overview.

Before you start: prepare the switches

LanTopoLog builds the map from the bridge MAC address table each switch exposes over SNMP. According to the official manual it reads the OIDs 1.3.6.1.2.1.17.4.3.1.2 (Bridge-MIB) and 1.3.6.1.2.1.17.7.1.2.2.1.2 (Q-Bridge-MIB). A switch that answers neither cannot have devices located behind it.

• Pick the right host. Run LanTopoLog from a machine in the management VLAN. The manual recommends connecting it as close as possible to the root switch of the map.
• Open the path. SNMP queries go to UDP 161 on every switch, and ping must work for monitoring.
• Use read-only access. On Cisco IOS, a read-only community restricted to the LanTopoLog host looks like this:

access-list 10 permit host 192.168.0.50
snmp-server community lt0pRO RO 10

Before blaming the tool, check that the forwarding table is readable from any Linux box with Net-SNMP installed:

snmpwalk -v2c -c lt0pRO 192.168.0.2 1.3.6.1.2.1.17.4.3.1.2

If you use SNMPv3 on Cisco, the manual warns that switches are typically not configured to return all Bridge-MIB data per VLAN. Allowing VLAN contexts for the SNMP group is the standard fix:

snmp-server group LTL v3 priv context vlan- match prefix
snmp-server user ltluser LTL v3 auth sha <auth-pass> priv aes 128 <priv-pass>

If your switches insist on SHA-512 authentication or AES-256 privacy, use the Java build (2.53.6j, currently beta). It needs a Java runtime; the developer offers a portable Microsoft OpenJDK and the program asks for the path to javaw.exe.

Install it

There is no installer. On Windows, extract the zip to a fixed folder such as C:Lantopolog and start lantopolog.exe; the program does not write to the registry or system folders. On Linux, extract the separate AMD64 build (2.52.9, a minor version behind Windows), make the lantopolog file executable and start it. It will ask you to install the snmp package (Net-SNMP utilities). The manual notes that the Linux version has limited functionality, so Windows is the better choice for a permanent monitor.

Step 1: find the switches

1. Enter the IP ranges where switches live, for example 192.168.0.*, 192.168.0.100-200 or 172.16.200-255.*.
2. Set the SNMP read community, or the SNMPv3 user and passwords, for each range.
3. To put a device without SNMP on the map (an unmanaged switch, a camera), enter its address with a plus sign, for example +192.168.0.1, and any name in the community field.
4. To skip a device, add its address with a minus sign: -192.168.0.99.
5. Click Run device discovery. Tick Also discover any SNMP device (AP…) if you want access points and similar devices on the map; Windows and Linux hosts and printers are not added this way.
6. Check that every expected switch is listed; test any missing one with an SNMP walk.

Step 2: collect SNMP data

Click Collect SNMP data and wait for the run to finish. LanTopoLog now reads the forwarding tables from every switch. In this step you can also tick a different switch as the root node of the map tree if the default choice is wrong.

Step 3: build and check the topology

1. Click Discover the Topology.
2. Compare the result with what you know about the cabling. Links the algorithm could not resolve are labelled xx.
3. Correct wrong or missing links under Options > Discovery > Edit connection table. Each line is one link; the switch higher in the tree goes on the left of the dash:

192.168.0.1 port 12 - 192.168.0.2 port 50
192.168.0.3 port ge-1/0/2 - 192.168.0.4 port Gi1/0/10

4. Run Discover the Topology again, then click Apply the New Topology. The saved map appears in the Network Browser tab.

For layout changes, such as making the router the root node, use the topology editor (Service > Edit the Topology). Links you draw there are added to the connection table automatically.

Reduce the number of unknown links

The developer states that the algorithm is not 100% reliable and recommends:

• Increase the MAC address aging time on the switches so entries stay in the table longer.
• Run discovery during working hours, when most computers are on.
• Enable LLDP (or CDP) on the switches.
• Fill the gaps with your own connection list.

Locate end devices on switch ports

Switch-to-switch links are half the job. Computer discovery finds MAC addresses, resolves them to IP addresses and works out the switch and port for each one. Schedule it under Options > Discovery (values below one hour are allowed, such as 0.5) or run it once from Action > Run Computer Discovery. A device whose MAC is not in any switch table is parked under a pseudo device as a temporary location until it shows up. Host names are re-resolved every two hours regardless of the schedule.

If names do not resolve, import them from a MAC;IP;hostname file via File > Import > IP, Hostname Import. The file must be ANSI, with ; or , as the delimiter:

F0-7B-CB-41-0B-9F;192.168.0.39;print-srv01

The manual names Nmap output (saved with -oN) and Advanced IP Scanner CSV results as ready-made sources.

Keep the map current and get alerts

• New switch, no full rescan. Add it in Step 1, find the switch it hangs off on the map and choose Add new switches here or Discover only the part of the map under this switch.
• Ping monitor. Under Options > Ping monitor, set the check interval and the number of failed attempts before a host is marked down. Switches are checked too; prefix a switch address with - in the host list to silence its alerts.
• Notifications. Email (SMTP servers that require an app password need one), Telegram via a bot token and chat ID, Syslog, a sound, or any program or script. You can also be notified when the spanning tree topology changes.
• Run it 24/7. Close the GUI, run ltl_svc_mgr.exe as administrator and install the service through the NSSM window. The service name must stay lantopolog, and the GUI and the service cannot run at the same time. The manual suggests a low-privilege account instead of Local System.
• Export. CSV lists of computers, switches, links, ports and VLANs; PDF through wkhtmltopdf; and a draw.io file that is always ready at %LOCALAPPDATA%LanTopoLog2Exporttop_map.xml.

When the map is stable, a full inventory tool such as Open-AudIT is a natural next step for hardware and software records.

Limitations

• Unregistered, the program stops showing VLAN IDs, traffic charts, search and web publishing some time after it starts, and shows “demo” instead of some data. Discovery itself keeps working.
• A licence covers one local network and is bound to up to three switches you choose at registration; other switches are not limited.
• Unmanaged switches and devices without SNMP must be added by hand.
• LanTopoLog is not a web server: publishing the map needs your own IIS or Apache.

FAQ

Is LanTopoLog free?

It is shareware (conditionally free). Topology discovery is unrestricted in the unregistered version; web publishing, VLAN display, traffic charts and search become unavailable some time after start until you register. The developer states that all updates are free.

What is LanTopoLog 2?

It is the current generation of the program. The Windows build is 2.53.13 (version 2.53 was released in December 2025), the Linux build is 2.52.9, and a Java build adds stronger SNMPv3 algorithms.

What are good LanTopoLog alternatives?

If you need monitoring of servers and services as well as maps, look at PRTG Freeware or NetCrunch (commercial). If you only need to know what is alive on a subnet, a scanner like Angry IP Scanner is quicker.

Is there a free switch port mapper?

LanTopoLog’s port mapping is part of discovery, which works without registration. Free scanners in our catalogue find hosts but do not show which switch port they use.

Can I export the network topology diagram?

Yes: to PDF (with wkhtmltopdf), to draw.io/diagrams.net, and to CSV tables. Exports can run on a schedule.

Last updated: 2 October 2026 · AdminHub Plus editorial team. Licence, version and platform details are checked against each developer's official documentation.