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 192.168.79.0/24 is divided into /27 subnets, numbered 1, 2, 3, … from the lowest address (subnet 1 starts at .0). What is the fourth octet x of the network address of subnet 2?
192.168.79.0/24/27subnet 2 (subnet 1 starts at .0)
x =
2.IPv4 subnettingHard
A host has the address 172.20.226.254/19. What is the third octet x of its subnet's broadcast address?
172.20.226.254/19
x =
3.Computer networksMedium
At which OSI layer does a switch normally operate?
- APhysical (layer 1)
- BTransport (layer 4)
- CNetwork (layer 3)
- DData link (layer 2)
4.Computer networksHard
A DNS cache poisoning attack aims to:
- ASteal Wi-Fi passwords
- BMake a resolver store a false IP address for a domain
- COverload a server with traffic so that it stops answering queries
- DDelete DNS servers
5.Number systemsMedium
Set the bits so they store −105 in 8-bit two's complement. Read as an unsigned number, what is the pattern's value U?
N = −1058 bit
U =
6.Number systemsHard
Set the bits to the 8-bit result of 0001 1000 >> 3. What is the result's value R in decimal?
0001 1000>> 3
R =
3Solutions
1.IPv4 subnetting
Answer: x = 32
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).
Block size
25 = 32
Subnet number 2
network: (2 − 1)·32 = 32
broadcast: 32 + 32 − 1 = 63
Answer
x = 32
2.IPv4 subnetting
Answer: x = 255
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).
Block size in the third octet
25 = 32
Subnet of this host
network: 172.20.224.0
broadcast: 172.20.255.255
Answer
x = 255
3.Computer networks
Answer: D · Data link (layer 2)
Why this is the answer
A standard switch learns MAC addresses and forwards frames at layer 2. Layer 3 switches also route.
Answer
D · Data link (layer 2)
4.Computer networks
Answer: B · Make a resolver store a false IP address for a domain
Why this is the answer
Poisoning inserts forged records into a resolver's cache. DNSSEC adds signatures so resolvers can verify records.
Answer
B · Make a resolver store a false IP address for a domain
5.Number systems
Answer: U = 151
Setting bits: each bit is worth a power of two (128, 64, …, 1). One hex digit is exactly four bits, and in 8-bit two's complement the leftmost bit is worth −128. Bitwise operations work on each bit position separately.
Invert and add one
105 = 0110 1001
inverted: 1001 0110
+1 → 1001 0111
Read it as unsigned
1001 0111₂ = 256 − 105 = 151
Answer
U = 151
6.Number systems
Answer: R = 3
Setting bits: each bit is worth a power of two (128, 64, …, 1). One hex digit is exactly four bits, and in 8-bit two's complement the leftmost bit is worth −128. Bitwise operations work on each bit position separately.
Bit by bit
Shifting right by 3 moves every bit 3 places to the right; bits that fall off the right end are lost and zeros come in on the left.
0001 1000 >> 3
= 0000 0011
Value of the result
0000 0011₂ = 3
Answer
R = 3
ExamProblems · examproblems.com/courses/computer-networks