IP addresses: How packets find their destination
An IP address identifies who you want to communicate with, but knowing the address alone is not enough to send a packet. Start with how to tell whether a destination is on the same network, how to calculate address-block sizes, and how DHCP and NAT work. Then explore enterprise address planning and public address management.
Start with how your PC identifies the destination, then follow the process of calculating, assigning, translating, and managing addresses.
Skip anything you already know. The links below cover the overall communication process and the principles of switching and routing.
When investigating an address question, read the device address, its mask, and the destination address together. Connect the article on deciding whether a destination is local with the subnet and CIDR explanation to understand the range represented by an address. Then separate how DHCP supplies an address from which address changes through NAT on the way to the Internet. Treating address assignment and address translation as different functions is a useful starting point.
Articles on enterprise address planning and public IP management extend the view from a single device to resources inside and outside an organization. For overlapping addresses, note who manages each range and use the linked tools where a calculation helps. A correct range calculation does not confirm permission to use the addresses or the device configuration. Return to the related switching and routing articles to see where those addresses are used during packet delivery.
Three sections in this chapter
Explore three sections, from understanding and calculating addresses to assigning, translating, and managing them.
How does my PC know if a destination is on the same network?
See how an endpoint uses bitwise AND with IPv4 addresses and a subnet mask to classify destinations as on-link or off-link and choose its ARP target.
Identify the destination and size the address block
Compare your address and the destination address using the same criteria, then calculate a subnet size that fits the number of devices you need.
Assign addresses to devices and connect them to the internet
Separate the steps: a device uses DHCP to receive an address and its gateway settings, then communicates with the internet through NAT.
Manage internal and public addresses without overlap
Plan the enterprise's private address space, then see why public address allocation and registration are separate responsibilities from advertising internet routes.
Background reading
These related articles are outside the core reading sequence.
- What happens between entering a URL and seeing a website? — See where addresses fit into the overall communication process
Explore further after the core articles
These related articles are outside the core reading sequence.
- Whose MAC address does my PC use to send data? — How an IP decision determines the destination MAC address
- How does a router choose where to send a packet? — How a router compares a destination address with available routes
- How do enterprise networks connect to branches, clouds and the internet? — Apply address planning to branch and cloud connections