IPアドレス確認

IPv4 vs IPv6

IPv4 (Internet Protocol version 4) is the longest-used addressing system on the Internet. It consists of four numbers (0-255 each) separated by dots (e.g., "192.0.2.1"). IPv4 can allocate approximately 4.3 billion addresses, but with the rapid growth of Internet users and IoT devices, these addresses are nearly exhausted.

IPv6 (Internet Protocol version 6) is the next-generation protocol developed to solve IPv4's address exhaustion problem. It consists of hexadecimal numbers separated by colons (e.g., "2001:0db8:0000:0000:0000:ff00:0042:8329"), providing virtually unlimited addresses (2^128). However, adoption in Japan is still around 50-60%, with IPv4 and IPv6 running in parallel.

Private vs Global IP Addresses

• Global IP Address: A unique address assigned on the Internet. This is what this tool displays. Assigned by your ISP (Internet Service Provider) and identifiable by any computer worldwide. Used for web browsing, email, and other Internet communications.
• Private IP Address: An address used only within limited networks such as corporate networks or home LANs. Reserved address ranges defined by RFC 1918: "10.0.0.0 - 10.255.255.255", "172.16.0.0 - 172.31.255.255", "192.168.0.0 - 192.168.255.255". Routers use NAT (Network Address Translation) to convert private IPs to global IPs for Internet communication.
• Localhost Address: "127.0.0.1" is a loopback address pointing to the computer itself. Used to access servers on your own machine without network connectivity.

IP Address Mechanism and Assignment

To communicate on the Internet, your computer (or smartphone) needs a global IP address. This address is assigned by your ISP. In most cases, ISPs operate DHCP (Dynamic Host Configuration Protocol) servers that automatically assign available addresses from a pool whenever your router connects to the Internet. With "dynamic" address assignment, different IP addresses may be assigned when the router restarts or after the DHCP lease time expires.

More specifically, IP addresses consist of a "network part" and a "host part". For example, in "192.168.1.100/24", the "/24" indicates the subnet mask - the first 24 bits are the network part, and the remaining 8 bits are the host part. This allows up to 254 devices to be connected within the "192.168.1.0" network. This hierarchical structure enables efficient routing (path selection) across the entire Internet.

Global IP addresses assigned by ISPs typically come in two service types: "static IP" and "dynamic IP". Dynamic IP is common and costs a few hundred yen per month, while static IP often costs over 1000 yen per month. If you don't mind your IP address changing frequently, dynamic IP is fine. However, if you "run a mail server", "host a web server", or "don't want access restrictions from non-static IP environments", you need a static IP contract.