Glossary · 2 min read

How Do IP Addresses Work?

Understand how IP addresses work, including IPv4 vs IPv6 formats, public vs private addresses, static vs dynamic assignment, NAT, and CIDR notation.

How Do IP Addresses Work?

An IP (Internet Protocol) address is a numerical identifier assigned to every device connected to a network that uses the Internet Protocol for communication. IP addresses serve two purposes: identifying the host and providing the location for routing packets to that host.

IPv4

IPv4 addresses are 32-bit numbers, written in dotted decimal notation as four octets separated by periods.

  • Format: 192.168.1.100
  • Range: 0.0.0.0 to 255.255.255.255
  • Total addresses: Approximately 4.3 billion (2^32)

IPv4 address exhaustion became a practical reality in the 2010s as the global pool of unallocated addresses was depleted. This drove adoption of NAT and the development of IPv6.

IPv6

IPv6 addresses are 128-bit numbers, written in colon-separated hexadecimal notation.

  • Format: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
  • Abbreviated: 2001:db8:85a3::8a2e:370:7334 (leading zeros dropped, consecutive zero groups replaced with ::)
  • Total addresses: 2^128 (approximately 3.4 x 10^38)

IPv6 eliminates the need for NAT and provides enough addresses for every device to have a globally unique address.

Public vs. Private Addresses

Public addresses are globally routable and unique across the internet. Your web server, mail server, and other internet-facing services use public IPs.

Private addresses (RFC 1918) are reserved for internal networks and are not routable on the public internet:

  • 10.0.0.0/8 (10.0.0.0 -- 10.255.255.255)
  • 172.16.0.0/12 (172.16.0.0 -- 172.31.255.255)
  • 192.168.0.0/16 (192.168.0.0 -- 192.168.255.255)

Devices on a private network access the internet through NAT.

Static vs. Dynamic

  • Static IP addresses are permanently assigned to a device. Servers (web, mail, DNS) require static IPs so that DNS records remain valid.
  • Dynamic IP addresses are assigned temporarily by a DHCP server and may change over time. Most residential internet connections use dynamic IPs, which is one reason ISPs block outbound port 25 for home users.

NAT (Network Address Translation)

NAT allows multiple devices on a private network to share a single public IP address. A NAT router translates between private and public addresses, tracking connections in a state table. While NAT conserves IPv4 addresses, it adds complexity and can interfere with protocols that embed IP addresses in their payloads.

CIDR Notation

CIDR (Classless Inter-Domain Routing) notation specifies an IP address and its associated network prefix length:

  • 192.0.2.0/24 means the first 24 bits are the network portion, leaving 8 bits for host addresses (256 addresses, 254 usable).
  • 10.0.0.0/8 covers the entire 10.x.x.x private range.

CIDR replaced the older classful addressing system (Class A/B/C) and is used in routing tables, firewall rules, and SPF records (ip4:192.0.2.0/24).

IP Addresses and Email

Your mail server's IP address is foundational to email deliverability. Receiving servers check the sending IP against blocklists, verify reverse DNS (PTR records), and evaluate IP reputation when making delivery decisions. Check your sending IP's standing with our IP reputation tool.

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