What is IP? IPv4, IPv6 and packet routing
Internet Protocol (IP) carries packets between networks using IP addresses. Today, IPv4 uses 32-bit addresses and IPv6 uses 128-bit addresses. IP provides addressing and routing, but does not by itself promise that every packet arrives, arrives in order or has protected contents. Other protocols and applications add the services they need.
How a packet crosses networks
An IP packet identifies its source and destination. Routers consult routing information to decide where to forward it next; the route may vary with network conditions and operator policy. IPv4 has a Time to Live field and IPv6 has a Hop Limit, both reduced as a packet is forwarded so that it cannot circulate indefinitely. If a packet is lost, a higher-layer protocol such as TCP may retransmit data when appropriate. IP itself does not provide the end-to-end reliability or sequencing described in the IPv4 specification.
IPv4 and IPv6 address formats
An IPv4 address is commonly written as four decimal numbers, such as 192.0.2.10. An IPv6 address uses groups of hexadecimal digits and permits a run of zero groups to be abbreviated, for example 2001:db8::10. These two sample ranges are reserved for documentation, not real public services. An address followed by /24 or /64 includes a network prefix length: that many leading bits describe the network portion. The larger IPv6 address space is already used on the Internet; IPv6 is not merely a future project. The IPv6 specification describes its header and Hop Limit.
Private addresses and public reachability
Private IPv4 addresses such as those in 192.168.0.0/16 can be reused in unrelated local networks. They commonly reach public IPv4 services through address translation at the network boundary. A globally scoped IPv6 address does not mean that outsiders can connect to a device: firewalls and operator policies may block inbound traffic. A public IP address also does not prove a person’s identity. It can be shared using NAT, assigned dynamically, or belong to a proxy or other intermediary.
A useful troubleshooting sequence
If a site will not open, first check the device’s address, its default gateway and DNS resolution. A domain can return both A and AAAA records, and a browser may try either address family. Test whether the problem occurs for one site, one protocol version or the entire network. Compare the destination address with the route and inspect local firewall rules before assuming the remote server is down. When describing the issue, record the address, prefix and exact time rather than an isolated screenshot of an IP number.