OpenCyber AcademyLesson map
Lesson 07. Map, example, EX053.
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
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:
- Place the printed addresses inside the private range of RFC 1918.
- Refuse a subnet mask the file does not state.
- 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
Lesson table; scroll horizontally to read all columns →
| 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 from192.168.1.41?Show marking criteria
Rubric: 3 = no mask is printed; the addresses sit in the RFC 1918 block192.168.0.0/16; private addressing is not inside isolation;.41stays on the printed triplet;; 2 = no invented mask, and the inside devices stay unseparated by that fact alone;; 1 = the triplet only;; 0 = none.