## Chapter 7 — One leading triplet is not a recorded prefix ### Lesson map Lesson 07. Map, example, EX053. ```mermaid flowchart TD R["Seven printed addresses"] --> P["Each begins 192.168.1"] P --> Q{"Is the mask in the file?"} Q -->|No| K["Do not invent a prefix length"] P --> N["Private space is not inside isolation"] ``` The addresses share a leading triplet. The mask is not printed. ### Logical drawing ```text 192.168.1.10 till-1 192.168.1.11 till-2 192.168.1.12 card-reader 192.168.1.20 office-pc 192.168.1.30 owner-phone 192.168.1.41 UNKNOWN 192.168.1.42 UNKNOWN mask: not in the file ``` ### Learning outcomes After this chapter, you should be able to: 1. Place the printed addresses inside the private range of RFC 1918. 2. Refuse a subnet mask the file does not state. 3. Separate "hidden from the public internet" from "separated from each other." ### 7.1 Private addressing explains the outside. It does not split the shop. RFC 1918 reserves `10.0.0.0/8`, `172.16.0.0/12`, and `192.168.0.0/16` for networks that are not routed on the public internet. Every address in the lease print begins `192.168.1`, so each one sits inside that third block. That fact supports chapter 1's plain description: the router faces the internet, and unrequested traffic from outside is what the router is said to resist. It does not, by itself, tell you the prefix length inside the shop. A leading triplet is compatible with a 24-bit local prefix, and it is also compatible with a wider or narrower mask. The script never prints a mask or a network statement. Writing `/24` on the note would be an assumption, not a reading. The inside question is the one chapter 1 already draws. Tills, the card reader, the office computer, the owner's phone, and customer phones share one local network, so nothing in that drawing stops them addressing each other. Network address translation, if this router performs it, rewrites inside sources as they leave toward the internet. The book does not print a public address, and it does not show a translation table, so this chapter does not claim that translation was observed. Even where translation is present, it is an outside hiding function. Two devices that already hold `192.168.1.10` and `192.168.1.41` can still exchange packets on the inside without any public address being involved. The decision stays the one Leo ranked first: separate customer Wi-Fi from business devices. Private addressing is not that separation. The two unknown rows, `.41` and `.42`, are on the same leading triplet as the tills. Their later names, speaker and scanner, do not move the printed numbers. ### 7.2 Worked example | Sentence | Evidence in the file | Decision | |---|---|---| | "These seven addresses are inside `192.168.0.0/16`." | RFC 1918 and the printed triplets | May stay | | "The mask is not printed." | No mask line | May stay | | "The shop is a /24 because the triplet matches." | Mask absent | Refuse | | "Private addressing already isolates the tills from the speaker." | Chapter 1's shared network | Refuse | > **Synthetic case.** No public address is added for Bramble Street. ### Errors and limits Hiding a shop from unsolicited internet traffic can be true while two inside devices still share a network. ### Apply - **EX053 — What may you write under the addresses?** All seven leases begin `192.168.1`. May you record a subnet mask, and does the private range isolate the tills from `192.168.1.41`? *Rubric:* 3 = no mask is printed; the addresses sit in the RFC 1918 block `192.168.0.0/16`; private addressing is not inside isolation; `.41` stays on the printed triplet;; 2 = no invented mask, and the inside devices stay unseparated by that fact alone;; 1 = the triplet only;; 0 = none. ### Continue studying — lesson 07 (chapter 07 of 20) [← Chapter 06](#ch-06) · [Course chapter index](../SUBJECTS.md) · [Chapter 08 →](#ch-08) Continue to the next authored lesson. ## Worked answers **EX053.** No mask is printed. The seven addresses sit in RFC 1918's block 192.168.0.0/16. That private range does not isolate 192.168.1.41 from the tills.