IPv4 subnetting
- Hosts per subnet
- 2(32 − prefix) − 2 (/31: 2, /32: 1)
- Block size
- 256 − (mask octet)
- Network address
- IP AND mask
- Broadcast address
- IP OR (NOT mask): all host bits set to 1
- Subnets from n borrowed bits
- 2n
Computer Networks: cram pack
Formula sheets, the key method steps for every topic and 6 practice problems, with full solutions at the back.
1Formulas and methods
Number systems
- Place values
- binary 2k, octal 8k, hex 16k
- To decimal
- Σ dk·bk
- From decimal
- divide by the base, read the remainders bottom-up
- Hex ↔ binary
- one hex digit = 4 bits
- Two’s complement
- invert the bits and add 1
2Practice problems
1.IPv4 subnettingMedium
The network 172.16.0.0/16 is divided into equal subnets with prefix /18. How many subnets N are there?
172.16.0.0/16/18
N =
2.IPv4 subnettingHard
A router has routes to these networks: 10.55.156.0/24, 10.55.157.0/24, 10.55.158.0/24, 10.55.159.0/24, 10.55.160.0/24, 10.55.161.0/24, 10.55.162.0/24, 10.55.163.0/24. What is the longest prefix length p of a single summary route that covers all of them?
10.55.156.0/24 … 10.55.163.0/24
p =
3.Computer networksMedium
What is the main difference between the HTTP methods GET and POST?
- AGET retrieves a resource, POST submits data in the request body
- BGET is encrypted, POST is not
- CGET uses UDP, POST uses TCP
- DGET deletes a resource, POST reads it from the server's database
4.Computer networksHard
Which of these addresses is a public (not RFC 1918 private) IPv4 address?
- A172.32.0.1
- B192.168.255.1
- C172.31.0.1
- D10.255.0.1
5.Number systemsMedium
The 8-bit pattern 1100 0000 stores a signed integer in two's complement. What is its value N in decimal?
1100 00008 bit
N =
6.Number systemsHard
A system must give every one of 1481 items its own binary code of the same length. What is the smallest number of bits b needed?
1481
b = bit
3Solutions
1.IPv4 subnetting
Answer: N = 4
IPv4 subnetting: an address has 32 bits. The prefix /p says how many leading bits identify the network; the remaining 32 − p bits number the hosts. A subnet has 2³²⁻ᵖ addresses, but the first (network address) and last (broadcast address) cannot be given to hosts, so 2³²⁻ᵖ − 2 hosts fit. Exceptions: a /31 (point-to-point link, RFC 3021) uses both of its 2 addresses for hosts, and a /32 is one single address (a single host or a loopback).
Borrowed bits
18 − 16 = 2
Number of subnets
N = 22 = 4
Answer
N = 4
Each /18 subnet has 214 − 2 = 16382 usable hosts.
2.IPv4 subnetting
Answer: p = 18
Route summarisation: a summary route a.b.c.0/p covers all networks whose first p bits match. Write the differing octet of the first and last network in binary; the number of common leading bits, plus the bits of the octets before it, is the longest summary prefix.
Compare the bits
156 = 10011100
163 = 10100011
common leading bits: 2
Prefix length
p = 16 + 2 = 18
Answer
p = /18
Summary route: 10.55.128.0/18 (it also covers 56 network(s) that are not in the list).
3.Computer networks
Answer: A · GET retrieves a resource, POST submits data in the request body
Why this is the answer
GET asks for a resource and should not change server state. POST sends data, such as a form, in the body for the server to process.
Answer
A · GET retrieves a resource, POST submits data in the request body
4.Computer networks
Answer: A · 172.32.0.1
Why this is the answer
172.16.0.0/12 runs from 172.16.0.0 to 172.31.255.255, so 172.32.0.1 lies outside it. The other three are in private ranges.
Answer
A · 172.32.0.1
5.Number systems
Answer: N = −64
Positional number systems: each digit is worth its value times a power of the base. In binary the places are worth 1, 2, 4, 8, 16, …; in hexadecimal 1, 16, 256, …, with A–F standing for 10–15. In 8-bit two's complement the leftmost bit is worth −128 instead of +128, which is how negative numbers are stored.
Add the place values
The other bits keep their normal positive values.
−128 + 64 = −64
Answer
N = −64
6.Number systems
Answer: b = 11 bit
Positional number systems: each digit is worth its value times a power of the base. In binary the places are worth 1, 2, 4, 8, 16, …; in hexadecimal 1, 16, 256, …, with A–F standing for 10–15. In 8-bit two's complement the leftmost bit is worth −128 instead of +128, which is how negative numbers are stored.
Powers of two
210 = 1024 < 1481
211 = 2048 ≥ 1481
Answer
b = 11
ExamProblems · examproblems.com/courses/computer-networks