What Do Those Numbers Mean? A Guide to Common Network Protocol Numbers

1. Introduction

Network protocols are the foundation of network communication, defining how data is transmitted. Each protocol is assigned a unique protocol number used in the IP header to ensure correct delivery and processing of packets.

2. Common Network Protocols and Their Numbers

2.1 Internet Control Message Protocol (ICMP) – Number 1

Purpose: Sends control messages such as error reports and network queries.

Example: The ping command uses ICMP to test reachability of another host.

2.2 Transmission Control Protocol (TCP) – Number 6

Purpose: Provides a reliable, connection‑oriented data transport service.

Example: Web services like HTTP and HTTPS rely on TCP.

2.3 User Datagram Protocol (UDP) – Number 17

Purpose: Offers an unreliable, connection‑less data transport service.

Example: DNS queries typically use UDP.

2.4 Generic Routing Encapsulation (GRE) – Number 47

Purpose: Encapsulates a wide range of network protocols over another protocol.

Example: VPNs often use GRE to encapsulate private traffic.

2.5 Virtual Router Redundancy Protocol (VRRP) – Number 112

Purpose: Provides high availability for routers or gateways.

Example: Keepalived implements VRRP for failover.

3. Importance of Protocol Numbers

Protocol numbers appear in the IP header and tell network devices how to interpret the following payload. Without them, devices cannot determine whether to process a packet with TCP, UDP, or another protocol, hindering correct routing and handling.

4. How to Find and Understand Protocol Numbers

Protocol numbers are managed and allocated by the Internet Assigned Numbers Authority (IANA). The complete list can be consulted on the IANA website, which helps with network troubleshooting and configuration.

5. Conclusion

Understanding network protocols and their assigned numbers is essential for reliable data transmission across diverse devices and networks, benefiting both network engineers and everyday users.