## Chapter 10 — 400 is why that address fails ### Lesson map Lesson 10. Map, example, EX056. ```mermaid flowchart TD V["198.51.100.400"] --> P["ip_address raises"] P --> D{"Digits only after dots are removed?"} D -->|Yes| I["print invalid and skip"] G["203.0.113.9"] --> OK["ip_address accepts"] ``` One call rejects the broken octet. A different value passes the same call. ### Logical drawing ```text rejected 198.51.100.400 octet 400 accepted as an address 203.0.113.9 not a lookup no DNS no route ``` ### Learning outcomes After this chapter, you should be able to: 1. Say which call rejects `198.51.100.400`. 2. Separate that rejection from a domain. 3. Refuse a network lookup this file does not make. ### 10.1 The library raises. The digit test then chooses the invalid line. `ipaddress.ip_address` accepts `203.0.113.9` and raises `ValueError` for `198.51.100.400`, because 400 is above 255. The except branch then removes dots and asks whether the rest is all digits. `19851100400` is all digits, so the script prints `invalid:` and does not append the pair. A domain fails the address call too, but the leftover characters are not all digits, so the domain is appended instead of rejected. The digit test is how this file tells a broken address from a name. It is not a reputation check. The header of the book already limits addresses to documentation ranges. `203.0.113.9` parses and sits in such a range. `198.51.100.400` does not parse, so it is not a host in `198.51.100.0/24`. The script never asks the network which of them exists. There is no DNS call in `lab_rapid.py`. Visiting either name is outside the lab, and this chapter does not add a lookup. | Value | Branch | |---|---| | `203.0.113.9` | Accepted by `ip_address` | | `198.51.100.400` | Raised, then classified invalid | | A domain with letters | Raised, then appended | | A DNS answer | Not requested | ### 10.2 Worked example | Offered claim | What the branch did | Decision | |---|---|---| | "400 cannot be an octet, so the line says invalid." | The raise plus the digit test | Keep | | "203.0.113.9 is kept by the same call." | No exception | Keep | | "The script resolved the name." | No DNS call | Refuse | | "The invalid address is a domain because it failed." | The digit test separated it | Refuse | > **Synthetic case.** Both values are classroom literals. ### Errors and limits Do not go and look the rejected value up. ### Apply - **EX056 — The octet.** Why is `198.51.100.400` invalid while `203.0.113.9` is kept, and does the script query DNS? *Rubric:* 3 = 400 is above 255 so ip_address raises; the digit test prints invalid and skips; 203.0.113.9 parses; there is no DNS call;; 2 = the octet fails and the other address parses;; 1 = one value;; 0 = none. ### Continue studying — lesson 10 (chapter 10 of 20) [← Chapter 09](#ch-09) · [Course chapter index](../SUBJECTS.md) · [Chapter 11 →](#ch-11) Continue to the next authored lesson. ## Worked answers **EX056.** 400 is above 255, so `ip_address` raises, the digit test prints invalid, and the pair is skipped. `203.0.113.9` parses. The script makes no DNS call.