Port 380 is assigned to is99s (TIA/EIA/IS-99 modem server), a protocol that enabled data transmission over early CDMA cellular networks.1 Both TCP and UDP traffic can use this port.
This port represents a forgotten era: when getting data over a cellular network meant connecting through a dedicated modem server, hearing virtual dial tones in the ether, and watching bits crawl at 14.4 kbps.
What IS-99 Does
IS-99 is a data services standard developed for IS-95 CDMA cellular networks (commonly known as cdmaOne).2 IS-95 was the first major 2G digital cellular technology, published in 1995 through collaboration between Qualcomm, Motorola, and AT&T.3
The cellular networks of the mid-1990s were designed for voice. IS-99 taught them to carry data—asynchronous data transmission and even Group-3 fax capability over the same wideband spread-spectrum cellular infrastructure that handled phone calls.4
Think of it as a translator. On one side: the packet-switched world of modems and early Internet access. On the other side: circuit-switched cellular voice networks. Port 380 was where the modem server listened, bridging these two worlds.
How It Worked
When a mobile device needed data, it would establish a connection to the IS-99 modem server on port 380. The server would handle the Radio Link Protocol (RLP) defined by IS-99 and related standards (IS-657, IS-707) to send packet data over IS-95 CDMA.5
The initial IS-95A standard provided 14.4 kbps data rates. Later, IS-95B allowed aggregating up to eight CDMA channels to reach 115.2 kbps—which felt fast in 1998.6
These weren't persistent data connections. You dialed in, used your minutes, and disconnected. The modem server on port 380 managed these sessions, converting between the cellular air interface and whatever data service you were trying to reach.
The Awkward Adolescence of Mobile Data
Port 380 represents cellular networking's growing pains. Before smartphones. Before 3G. Before the idea that a phone could maintain a persistent Internet connection seemed remotely economical or practical.
This was when "mobile data" meant:
- Connecting your Palm Pilot via a cellular modem
- Checking email at 14.4 kbps
- Paying by the minute
- Hearing the modem handshake in your head
The IS-99 server was infrastructure—invisible to users, essential to operators. It sat in the cellular network, listening on port 380, ready to authenticate modem connections and route data packets through a system designed for voice.
What Replaced It
CDMA evolved rapidly. IS-95B gave way to cdma2000 and 1xRTT, which brought always-on packet data. Then EVDO (Evolution-Data Optimized) arrived, and suddenly the idea of a dedicated modem server became obsolete.3
Modern cellular networks don't use port 380. LTE and 5G are packet-switched from the ground up. There's no modem handshake, no dial-up paradigm, no server listening on this port.
But in the late 1990s, this was the path. The IS-99 modem server on port 380 was how you got your laptop online through a CDMA phone, how you sent a fax from a moving car, how the cellular network learned to speak data.
Current Status
Port 380 remains officially assigned to IS-99 in the IANA registry,1 but the protocol is obsolete. IS-95 CDMA networks have been decommissioned. The technology moved on.
You won't find anything listening on port 380 in modern networks. This port is a historical marker—a reminder that every revolution in connectivity starts awkward, expensive, and slower than you want it to be.
Security Considerations
Since IS-99 is obsolete and IS-95 networks no longer operate, port 380 should not be open on any modern system. If you find this port listening:
- Verify what's actually using it (see below)
- It's likely misconfigured legacy software or something else incorrectly using this port number
- Block it unless you have a specific, documented reason not to
How to Check What's Using Port 380
On Linux/macOS:
On Windows:
Related Ports
- Port 379: is99c (IS-99 modem client) - the client side of this same system
- Port 22: SSH - what eventually replaced dial-up modem access for remote connections
- Port 80/443: HTTP/HTTPS - the services IS-99 users were desperately trying to reach at 14.4 kbps
Frequently Asked Questions
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