Port 1991Cisco STUN Priority 2

Cisco STUN Priority 2 traffic uses TCP or UDP port 1991.

transport: tcp · udp
in transit: cleartext
assignment: official
risk: 5/10
lookups: 0

also known as stun-p2

Technical Details

what runs on :1991

Cisco STUN encapsulates serial traffic, such as SDLC or other supported serial link protocols, inside Cisco-to-Cisco IP tunnel traffic. It can use TCP or UDP; port 1991 is the registered Priority 2 service in the Cisco STUN family, alongside separate priority assignments on nearby ports. The protocol is Cisco-specific rather than RFC 5389 Session Traversal Utilities for NAT, and normal deployments do not provide encryption.

Security Information

exposure of :1991

risk score

5/ 10caution

worth attention. how exposed you are depends on configuration — don't leave it reachable from the internet without a reason.

network services averages 2.6 across 1,653 ports — this one sits 2.4 above.

in transit

cleartext

credentials and payloads are readable by anyone on path

assignment

official

registered with iana for this service — scanners fingerprint it by number

reachable over

tcp · udp

udp is connectionless — source addresses are trivially spoofed and it is a reflection favourite

security overview

Internet exposure is generally inappropriate: an exposed listener may permit unauthorized tunnel establishment or access to attached serial devices if Cisco STUN configuration and peer restrictions are weak. Traffic is normally cleartext and can expose or transport sensitive control traffic, so restrict the port to known router peers with filtering and use a protected network or encrypted tunnel.

Record

iana registry entry

port: :1991
service: Cisco STUN Priority 2
transport: TCPUDP
encryption: cleartext
assignment: iana registered
risk: 5/10caution
category: Network Services
usage: This port is used by Cisco STUN (Serial Tunnel), which transports serial-protocol traffic between Cisco routers across an IP network. Port 1991 is the Priority 2 member of Cisco's STUN port assignments and is encountered on IOS routers configured for serial tunneling, not as the modern NAT-traversal STUN protocol.
lookups: 0rank 993
added: 2026-08-29
updated: 2026-09-22
references: IANA service name registry — port 1991

Something wrong?

This record is maintained by the engineers who use port 1991. Every field on it is editable — change what's wrong, say how you know, and a reviewer publishes it, usually within a minute. Last touched 2026-09-22.

01edit any field on a page of its own
02a model checks it for accuracy and spam
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edit this record →back to the registry

Related Ports

all 1,653 in network services →

the 8 most looked-up other ports in network services — 1,653 ports carry that label.

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:9080	Groove RPC	TCPUDP	Web Services	caution	70.8k
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:5938	TeamViewer	TCPUDP	Remote Access	caution	65.0k
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:3233	WhiskerControl Protocol	TCPUDP	Network Services	caution	61.9k
:3268	Microsoft Global Catalog (GC)	TCPUDP	Security	caution	54.6k
:135	Microsoft EPMAP	TCPUDP	Security	caution	51.9k
:3128	Tatsoft Default HTTP Proxy	TCP	Web Services	caution	46.8k

risk mix of the 8 listed

caution100%

0 of 8 encrypted

Port :1991Cisco STUN Priority 2

Technical Details

Security Information

Related Ports

Port 1991Cisco STUN Priority 2