The Internet Protocol version 4 (IPv4) (RFC791) there is a field, called "Protocol", to identify the next level protocol. This is an 8 bit field. In Internet Protocol version 6 (IPv6) (RFC1883 and RFC2460) this field is called the "Next Header" field.

Protocol Numbers

•  0 HOPOPT IPv6 Hop-by-Hop Option
• 1 ICMP Internet Control Message
• 2 IGMP Internet Group Management
• 3 GGP Gateway-to-Gateway
• 4 IP IP in IP (encapsulation)
• 5 ST Stream
• 6 TCP Transmission Control
• 7 CBT CBT
• 8 EGP Exterior Gateway Protocol
• 9 IGP any private interior gateway (used by Cisco for their IGRP)
• 10 BBN-RCC-MON BBN RCC Monitoring
• 11 NVP-II Network Voice Protocol
• 12 PUP PUP
• 13 ARGUS ARGUS
• 14 EMCON EMCON
• 15 XNET Cross Net Debugger
• 16 CHAOS Chaos
• 17 UDP User Datagram
• 18 MUX Multiplexing
• 19 DCN-MEAS DCN Measurement Subsystems
• 20 HMP Host Monitoring
• 21 PRM Packet Radio Measurement
• 22 XNS-IDP XEROX NS IDP
• 23 TRUNK-1 Trunk-1
• 24 TRUNK-2 Trunk-2
• 25 LEAF-1 Leaf-1
• 26 LEAF-2 Leaf-2
• 27 RDP Reliable Data Protocol
• 28 IRTP Internet Reliable Transaction
• 29 ISO-TP4 ISO Transport Protocol Class 4
• 30 NETBLT Bulk Data Transfer Protocol
• 31 MFE-NSP MFE Network Services Protocol
• 32 MERIT-INP MERIT Internodal Protocol
• 33 SEP Sequential Exchange Protocol
• 34 3PC Third Party Connect Protocol
• 35 IDPR Inter-Domain Policy Routing Protocol
• 36 XTP XTP
• 37 DDP Datagram Delivery Protocol
• 38 IDPR-CMTP IDPR Control Message Transport Proto
• 39 TP++ TP++ Transport Protocol
• 40 IL IL Transport Protocol
• 41 IPv6 Ipv6
• 42 SDRP Source Demand Routing Protocol
• 43 IPv6-Route Routing Header for IPv6
• 44 IPv6-Frag Fragment Header for IPv6
• 45 IDRP Inter-Domain Routing Protocol
• 46 RSVP Reservation Protocol
• 47 GRE General Routing Encapsulation
• 48 MHRP Mobile Host Routing Protocol
• 49 BNA BNA
• 50 ESP Encapsulating Security Payload
• 51 AH Authentication Header
• 52 I-NLSP Integrated Net Layer Security TUBA
• 53 SWIPE IP with Encryption
• 54 NARP NBMA Address Resolution Protocol
• 55 MOBILE IP Mobility
• 56 TLSP Transport Layer Security Protocol using Kryptonet key management
• 57 SKIP SKIP
• 58 IPv6-ICMP ICMP for IPv6
• 59 IPv6-NoNxt No Next Header for IPv6
• 60 IPv6-Opts Destination Options for IPv6
• 61 any host internal protocol
• 62 CFTP CFTP
• 63 any local network
• 64 SAT-EXPAK SATNET and Backroom EXPAK
• 65 KRYPTOLAN Kryptolan
• 66 RVD MIT Remote Virtual Disk Protocol
• 67 IPPC Internet Pluribus Packet Core
• 68 any distributed file system
• 69 SAT-MON SATNET Monitoring
• 70 VISA VISA Protocol
• 71 IPCV Internet Packet Core Utility
• 72 CPNX Computer Protocol Network Executive
• 73 CPHB Computer Protocol Heart Beat
• 74 WSN Wang Span Network
• 75 PVP Packet Video Protocol
• 76 BR-SAT-MON Backroom SATNET Monitoring
• 77 SUN-ND SUN ND PROTOCOL-Temporary
• 78 WB-MON WIDEBAND Monitoring
• 79 WB-EXPAK WIDEBAND EXPAK
• 80 ISO-IP ISO Internet Protocol
• 81 VMTP VMTP
• 82 SECURE-VMTP SECURE-VMTP
• 83 VINES VINES
• 84 TTP TTP
• 85 NSFNET-IGP NSFNET-IGP
• 86 DGP Dissimilar Gateway Protocol
• 87 TCF TCF
• 88 EIGRP EIGRP
• 89 OSPFIGP OSPFIGP
• 90 Sprite-RPC Sprite RPC Protocol
• 91 LARP Locus Address Resolution Protocol
• 92 MTP Multicast Transport Protocol
• 93 AX.25 AX.25 Frames
• 94 IPIP IP-within-IP Encapsulation Protocol
• 95 MICP Mobile Internetworking Control Protocol
• 96 SCC-SP Semaphore Communications Sec. Protocol
• 97 ETHERIP Ethernet-within-IP Encapsulation
• 98 ENCAP Encapsulation Header
• 99 any private encryption scheme
• 100 GMTP GMTP
• 101 IFMP Ipsilon Flow Management Protocol
• 102 PNNI PNNI over IP
• 103 PIM Protocol Independent Multicast
• 104 ARIS ARIS
• 105 SCPS SCPS
• 106 QNX QNX
• 107 A/N Active Networks
• 108 IPComp IP Payload Compression Protocol
• 109 SNP Sitara Networks Protocol
• 110 Compaq-Peer Compaq Peer Protocol
• 111 IPX-in-IP IPX in IP
• 112 VRRP Virtual Router Redundancy Protocol
• 113 PGM PGM Reliable Transport Protocol
• 114 any 0-hop protocol
• 115 L2TP Layer Two Tunneling Protocol
• 116 DDX D-II Data Exchange (DDX)
• 117 IATP Interactive Agent Transfer Protocol
• 118 STP Schedule Transfer Protocol
• 119 SRP SpectraLink Radio Protocol
• 120 UTI UTI
• 121 SMP Simple Message Protocol
• 122 SM SM
• 123 PTP Performance Transparency Protocol
• 124 ISIS over IPv4
• 125 FIRE
• 126 CRTP Combat Radio Transport Protocol
• 127 CRUDP Combat Radio User Datagram
• 128 SSCOPMCE
• 129 IPLT
• 130 SPS Secure Packet Shield
• 131 PIPE Private IP Encapsulation within IP
• 132 SCTP Stream Control Transmission Protocol
• 133 FC Fibre Channel
• 134 RSVP-E2E-IGNORE
• 135-254 Unassigned
• 255 Reserved