IoT SIM form factors
| Form factor | Size | Use case | Notes |
|---|---|---|---|
| 2FF (Mini SIM) | 25 x 15 mm | Legacy equipment | Rarely used in new IoT designs |
| 3FF (Micro SIM) | 15 x 12 mm | Some industrial routers | Uncommon in new designs |
| 4FF (Nano SIM) | 12.3 x 8.8 mm | Most modern routers | Standard for Teltonika RUT/TRB series |
| MFF2 (soldered) | 5 x 6 mm | Harsh environments | Vibration-resistant, IP-rated enclosures, -40 to +105°C |
Multi-network SIM cards
A multi-network SIM - sometimes called a roaming SIM or steered roaming SIM - can connect to multiple UK operators. The SIM carries roaming agreements with EE, Vodafone, Three and O2. When the primary operator is unavailable, the SIM automatically connects to the next available network. For IoT commissioning, a multi-network SIM is essential for running a proper multi-operator sweep with CellTester - it allows the automated test to lock to each network in turn without manually swapping SIMs.
Private APN and public IP SIMs
Standard consumer SIMs use a shared public APN with NAT - the device gets a private RFC1918 IP address, not a publicly routable one. For IoT deployments requiring inbound connections (remote access, SCADA, management platforms), you need either a public IP SIM or a private APN.
A public IP SIM gives the device a publicly routable IP address. This enables direct inbound connections but also exposes the router to the public internet. Routers deployed on public IP SIMs must have firewall rules and changed default credentials - and should be security scanned before going live. CellTester includes a CVE and open port scan on the WAN IP as part of the commissioning workflow.
A private APN gives devices IP addresses in a dedicated range accessible only via a private network or VPN. This is the more secure approach for enterprise IoT deployments.