A common datagram across networks
Different links can use different local transmission formats. IP carries upper-layer data inside a common datagram containing addresses and control information, while each link transports that datagram locally. Routers select forwarding steps using destination and routing information, allowing communication across networks built with different physical technologies.
Best-effort delivery of individual packets
IP treats datagrams individually and does not guarantee arrival or arrival order. Failures, congestion, and lifetime limits can cause packets to be discarded. Keeping this service general permits different upper-layer transports: applications needing ordered reliable delivery can use TCP rather than expecting IP itself to promise successful completion.
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Differences between IPv4 and IPv6
IPv4 uses 32-bit addresses; IPv6 expands them to 128 bits and changes header organization and extension mechanisms. This is not merely a longer copy of the old header: rules such as fragmentation differ as well. Hosts and network equipment need appropriate support to interpret the different packet formats.
Addressing, routing, and security
An IP address is addressing information rather than a person’s permanent identity or a website name. Routing selects where a packet goes next; other protocols handle naming, transport, and application exchange. Header checks and lifetime limits are not authentication, and a correctly formatted IP packet need not contain trusted or encrypted content.