The /25 ends at .127 | Rapid course Authorized offensive security and red team

Lesson map

Lesson 07. Map, example, EX053.

flowchart TD
  N["192.0.2.0/25"] --> S["128 addresses"]
  S --> H[".0 through .127"]
  H --> X[".128 starts the next block"]
  X --> M[".130 is not a member"]

Prefix 25 leaves seven bits. Seven bits count 128 addresses, and the block stops at .127.

Logical drawing

192.0.2.0/25
  first address  192.0.2.0
  last address   192.0.2.127
  count          128
192.0.2.130
  not a member

Learning outcomes

After this chapter, you should be able to:

Name the first and last addresses of 192.0.2.0/25.
Place 192.0.2.130 in the following block.
Refuse the idea that "130 is close, so the lab should have allowed it."

7.1 Prefix length is a count of bits, not a guess about nearby hosts.

ip_network("192.0.2.0/25") builds one network. A prefix of 25 means the first 25 bits are fixed and the last 7 bits vary. Two to the power 7 is 128, so the object covers 192.0.2.0 through 192.0.2.127 inclusive. The next address, 192.0.2.128, is the start of 192.0.2.128/25. 192.0.2.130 is that later block plus two. Membership is false.

The script never prints .127 or .128. Those two addresses are how you check the object, not extra rows in the output. .5 and .10 fall inside the 128. .130 does not. Closeness of the decimal is not the test ip in n uses.

Lesson table; scroll horizontally to read all columns →

Address	Inside this object?	Why
`192.0.2.0`	Yes	First address of the object
`192.0.2.127`	Yes	Last address of the object
`192.0.2.128`	No	First address of the next `/25`
`192.0.2.130`	No	Two past that boundary

7.2 Worked example

Lesson table; scroll horizontally to read all columns →

Offered claim	Check	Decision
"`.130` is outside because 130 is greater than 127."	The last address is `.127`	Keep
"The script printed a boundary row for `.127`."	No such line exists	Refuse
"A `/24` was what the file stored."	The literal is `/25`	Refuse

Synthetic case. The boundary is a property of the network object. It is not a fifth printed line.

Errors and limits

Do not widen the literal to a /24 so .130 becomes a member.

Apply

EX053 — Block edge. Where does 192.0.2.0/25 stop, and why is 192.0.2.130 outside it?
Show marking criteria
Rubric: 3 = the object holds 128 addresses, .0 through .127; .128 starts the next block; .130 is past that edge; the script does not print the edge;; 2 = .127 is the last member and .130 is not;; 1 = one address;; 0 = none.