LanTopoLog

LanTopoLog is a small, portable tool that discovers the physical (layer 2) topology of an Ethernet network over SNMP. It shows which switch port every switch, server and PC is plugged into, then keeps watching those links. It is aimed at network admins who inherited an undocumented LAN and want a map without deploying a full monitoring suite. If you are searching for a LanTopoLog download, the current program is LanTopoLog 2, available as a portable Windows build and a Linux build.

LanTopoLog is shareware (conditionally free). Topology discovery works without limits in the unregistered version, so you can map a network and check the result before deciding anything. Some extras, such as web publishing, VLAN display, traffic charts and search, stop working some time after the program starts unless it is registered.

LanTopoLog at a glance

What it does

• Discovers physical topology automatically from switch forwarding tables (Bridge-MIB) read over SNMP (community string or SNMPv3).
• Works with switches from any vendor, provided they answer SNMP.
• Draws a searchable topology map and resolves each switch port to the MAC and IP addresses behind it.
• Shows the VLANs assigned to ports and reports spanning tree changes.
• Monitors links and hosts with ping and port traffic charts.
• Sends alerts by email, Telegram or Syslog.
• Publishes the map to a web server, from which you can also trigger discovery remotely through a browser.
• Can run as a Windows service for continuous monitoring.

How sysadmins use it

Prepare switches for discovery

LanTopoLog only needs read access. On a Cisco IOS switch, a read-only community limited to the LanTopoLog host looks like this:

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

With SNMPv3, Cisco switches often hide per-VLAN Bridge-MIB data. The usual fix is to allow VLAN contexts for the group:

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>

Check from any Linux box that the forwarding table is readable before you blame the tool:

snmpwalk -v2c -c lt0pRO 192.168.0.2 1.3.6.1.2.1.17.4.3.1.2

Map the network in three steps

1. Step 1: enter the IP ranges where switches live (for example 192.168.0.* or 10.0.1.100-200) and the SNMP community or v3 credentials for each range, then click Run device discovery. Tick Also discover any SNMP device (AP…) to include access points and similar devices.
2. Step 2: click Collect SNMP data and wait while LanTopoLog reads the forwarding tables from every switch.
3. Step 3: click Discover the Topology, review the result in the Network Browser tab, correct any links with Edit connection table, and click Apply the New Topology.

A device that does not speak SNMP, such as an unmanaged switch or a camera you want on the map, can be added in Step 1 by entering its IP address with a + in front.

Run it as a 24/7 monitor

1. Unzip the portable build to a fixed folder such as C:Lantopolog, run the GUI once, and finish discovery, notifications and options.
2. Close the GUI and run ltl_svc_mgr.exe as administrator.
3. Click Install Service and confirm in the NSSM installer window. The service name must stay lantopolog.
4. To change settings later, stop the service first. The service and the GUI cannot run at the same time.

Install and first run

Windows. No installer. Unzip the archive and run it. The program does not write to the registry or system folders, so it fits on an admin USB stick. Running it from a host in the management VLAN saves a lot of firewall work.

Linux. A separate portable AMD64 build is offered (currently 2.52.9, one minor release behind Windows). Unpack it and run it from its folder.

Java build. 2.53.6j is a beta that runs on Microsoft OpenJDK and adds SNMPv3 authentication up to SHA-512 and AES-256 privacy. Use it only if your switches require those algorithms.

Network gotchas. SNMP uses UDP 161 from the LanTopoLog host to each switch. Switch ACLs or management-plane policies that silently drop SNMP are the most common reason a switch is missing. Ping must also be allowed for monitoring. The web feature needs your own web server (for example IIS or Apache with PHP). LanTopoLog writes HTML/PHP files into it and has no built-in web server or login.

Limitations

• Topology quality depends on switch SNMP data. Unmanaged switches cannot be queried and have to be added to the map by hand.
• Large multi-VLAN Cisco networks with SNMPv3 need the extra context configuration shown above.
• The unregistered version hides VLAN IDs, traffic charts, search and web publishing after a while, and shows “demo” instead of real data in places.
• A licence covers one local network and is bound to up to three switches you select at registration.
• The interface is functional but dated, and documentation is a concise online manual.
• No macOS build.

LanTopoLog vs alternatives

NetCrunch and PRTG are full monitoring platforms that also draw maps. They are heavier to deploy, but they cover servers, services and alerting far beyond layer 2. SoftPerfect Network Scanner finds hosts, shares and open ports quickly, but it does not show which switch port a device is plugged into. LanTopoLog’s niche is narrow and useful: an accurate port-level map from a 2 MB portable tool.

FAQ

Is LanTopoLog free?

It is shareware (conditionally free). Topology discovery works without limits unregistered; web publishing, VLAN display, traffic charts and search are time-limited until you register.

Is there LanTopoLog for Linux?

Yes. A portable Linux AMD64 build is published next to the Windows one, usually a minor version behind.

Is LanTopoLog safe?

It is portable and does not modify the registry or system folders, and it only needs read-only SNMP access. Give it a read-only community or v3 user, never a read-write one.

Does LanTopoLog need agents on computers?

No. It reads switch tables over SNMP and uses ping. Nothing is installed on the hosts being mapped.

Is LanTopoLog on GitHub or open source?

No. It is closed-source software from an independent developer, distributed through its own website.

Why are some switches missing from the map?

Usually SNMP is blocked or the community string is wrong. Test the switch with an SNMP walk, then check per-VLAN access if you use SNMPv3 on Cisco.

What are good LanTopoLog alternatives?

NetCrunch, PRTG and Checkmk for broader monitoring with maps. If you only need host discovery, a network scanner is quicker.

Last checked against official sources: 1 October 2026 (developer website: lantopolog.com). Versions and licence terms change — confirm on the developer's site before deploying in production.