# IP Tools — Tips & Tricks > Tricks for the IP Tools: understanding CIDR in your head, subnetting strategy, ULA vs. GUA, common mistakes, and combining with other JPKCom tools. Source: https://www.jpkc.com/db/en/tools/ip/tips/ Back to the overview: [IP Tools](https://www.jpkc.com/db/en/tools/ip/) · Open the tool live: [www.jpkc.com/tools/ip/](https://www.jpkc.com/tools/ip/) The [manual](https://www.jpkc.com/db/en/tools/ip/manual/) explains every tab, the [examples](https://www.jpkc.com/db/en/tools/ip/examples/) show the calculations. This page is about the understanding behind them: how to read CIDR in your head, how to plan a network sensibly, when to use ULA over GUA, and which mistakes keep happening. ## Understand CIDR instead of memorizing it The prefix after the slash is just the **number of network bits**. Everything else follows from it: - **Host bits = 32 − prefix**, and therefore **total addresses = 2 to the power of host bits**. A `/24` has 8 host bits → 256 addresses, a `/26` has 6 → 64, a `/30` has 2 → 4. - **Usable hosts = total addresses − 2** (the network and broadcast addresses come off). Exceptions: a `/31` has exactly **2** usable hosts per RFC 3021 (for point-to-point links), a `/32` is a **single host**. - **Each bit halves.** From `/24` to `/25` the block halves (256 → 128), from `/25` to `/26` again (128 → 64). The other way, each bit fewer doubles it. If you're unsure: the **Binary Breakdown** in the Subnet Calc tab colors network and host bits differently, so you literally see where the prefix ends. And the **IPv4 CIDR Quick Reference** in the Reference tab lists the mask and host count for every prefix; the **Common** button hides the exotic ones. ## Subnetting strategy - **Plan from the largest requirement.** Count the hosts you need per segment, round up to the next power of two (plus the two reserved addresses), and pick the matching prefix. 100 hosts need a `/25` (126 usable), not a `/26` (62). - **Use VLSM instead of cutting everything the same size.** With Variable Length Subnet Masking you give large segments a `/24` and point-to-point links a `/30` (or `/31`). The **Subnet Splits** in the Subnet Calc tab show you directly which narrower prefixes a block divides into and how many sub-networks result. - **Aggregate where you can.** Several adjacent networks can often be summarized into a shorter prefix (supernetting). That's exactly what **Range → CIDRs** is for: enter the overall range and see how few blocks it takes to cover it. - **`/31` for router links.** On point-to-point links, a `/31` (RFC 3021) saves a whole address over a `/30`. The tool calculates both correctly. ## ULA vs. GUA — which IPv6 address for what - **GUA (Global Unicast, `2000::/3`)** are publicly routable — the normal case in the IPv6 internet. They usually arrive via prefix delegation from your provider (typically a `/48` or `/56`). - **ULA (Unique Local, `fd00::/8`)** are the private counterpart, comparable to RFC 1918 addresses in IPv4. They aren't routed to the internet and suit internal services, lab and test networks, and addresses meant to stay stable independent of the ISP assignment. - **The real ULA advantage is the random Global ID.** Because the ULA generator draws the 40-bit Global ID cryptographically at random, your prefix is statistically unique worldwide. Two networks generated this way can be merged without an address conflict — unlike IPv4, where the same `192.168.0.0/16` collides everywhere. - **Generate your ULA prefix once and keep it.** The whole point is stability: roll a new one each time you need it and you lose exactly the uniqueness that makes ULA worthwhile. Generate the `/48` once, document it, and split it into your `/64` segments via the 16-bit Subnet ID. ## Common mistakes - **Assigning the network or broadcast address to a host.** The first and last address of a subnet are reserved. The calculator shows you with **First/Last Usable Host** exactly the range you may actually hand out. - **Entering an invalid "mask."** A subnet mask has to be contiguous (`255.255.255.192`), not full of holes (`255.0.255.0`). For a non-contiguous mask the Subnet Calc tab reports invalid input — that's not a bug, it's correct behavior. - **Forgetting that "IP only" assumes a `/24`.** If you enter an address with no suffix in the Subnet Calc tab, the tool calculates with `/24`. For a different network, state the prefix or mask explicitly. - **Confusing IPv6 and IPv4 when it comes to ranges.** IPv6 has **no broadcast** — multicast (`ff00::/8`) takes over that job. So don't look for a broadcast address for IPv6 in the subnet calculator; the calculator is designed for IPv4 anyway. - **Using documentation ranges in production.** `192.0.2.0/24`, `198.51.100.0/24`, `203.0.113.0/24`, and `2001:db8::/32` are explicitly reserved for examples. The Reference tab flags them — use them for documentation, never for real devices. ## Combining with other JPKCom tools - **From the IP to the domain.** Once you've classified an address or planned a subnet, the next step often goes one level up — with the **[DNS, SSL, Redirect & URL Tools](https://www.jpkc.com/db/en/tools/dns-ssl-redirect-url/)** you check DNS records, certificates, and redirects for the domain behind it. - **Placing your own location.** The **My IP** tab shows your address and detected proxy headers; for the full picture of your connection and browser, the **[Server & Browser Info](https://www.jpkc.com/db/en/tools/info/)** rounds it out. --- More context: the [overview](https://www.jpkc.com/db/en/tools/ip/) for the big picture, the [manual](https://www.jpkc.com/db/en/tools/ip/manual/) for every field, and the [examples](https://www.jpkc.com/db/en/tools/ip/examples/) for the step-by-step calculations. You can try everything directly in the [tool](https://www.jpkc.com/tools/ip/).