What IP protocol number is used in the protocol field of an IPv4 header, when IPv4 is used to tunnel IPv6 packets?
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The IP protocol number is an 8-bit field in the IPv4 header, which identifies the protocol that is being transported in the payload of the IPv4 packet. When IPv4 is used to tunnel IPv6 packets, the protocol number used in the IPv4 header is 41.
IPv6 packets are typically larger than IPv4 packets, and some networks may not be able to support the larger packet size. To overcome this limitation, IPv6 packets can be encapsulated within IPv4 packets using a technique called IPv6 over IPv4 tunneling. This allows IPv6 packets to be transmitted over an IPv4 network, without requiring upgrades to the network infrastructure.
In an IPv6 over IPv4 tunnel, the original IPv6 packet is encapsulated within an IPv4 packet. The IPv4 header contains the source and destination IP addresses of the tunnel endpoints, while the IPv6 packet is used as the payload of the IPv4 packet. The protocol number 41 is used in the protocol field of the IPv4 header to indicate that the payload is an IPv6 packet.
To summarize, when IPv4 is used to tunnel IPv6 packets, the IP protocol number used in the protocol field of the IPv4 header is 41.