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 192.168.210.0/24 is divided into /29 subnets, numbered 1, 2, 3, … from the lowest address (subnet 1 starts at .0). What is the fourth octet x of the broadcast address of subnet 11?
192.168.210.0/24/29subnet 11 (subnet 1 starts at .0)
x =
2.IPv4 subnettingHard
A host has the address 10.20.154.193/29. What is the fourth octet x of its network address?
10.20.154.193/29
x =
3.Computer networksMedium
Why does a switch sometimes send a frame out of all ports except the one it came in on?
- AThe destination MAC address is not yet in its address table
- BThe switch is acting as a router
- CThe frame has a wrong IP checksum and must be checked by every host
- DThe frame is too large for one port
4.Computer networksHard
Why does a browser warn when a site presents a self-signed certificate?
- AThe site must be using HTTP instead of HTTPS
- BThe certificate is always expired
- CNo trusted certificate authority vouches that the key belongs to that site
- DSelf-signed certificates use no encryption at all, so the traffic is sent in clear text
5.Number systemsMedium
Set the bits so they store −108 in 8-bit two's complement. Read as an unsigned number, what is the pattern's value U?
N = −1088 bit
U =
6.Number systemsHard
Set the bits to the 8-bit result of 1001 1010 << 3. What is the result's value R in decimal?
1001 1010<< 3
R =
3Solutions
1.IPv4 subnetting
Answer: x = 87
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
23 = 8
Subnet number 11
network: (11 − 1)·8 = 80
broadcast: 80 + 8 − 1 = 87
Answer
x = 87
2.IPv4 subnetting
Answer: x = 192
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
23 = 8
Subnet of this host
network: ⌊193/8⌋·8 = 192
broadcast: 192 + 8 − 1 = 199
usable hosts: 193 … 198
Answer
x = 192
3.Computer networks
Answer: A · The destination MAC address is not yet in its address table
Why this is the answer
For an unknown unicast address the switch floods the frame. When the destination replies, the switch learns its port from the source MAC.
Answer
A · The destination MAC address is not yet in its address table
4.Computer networks
Answer: C · No trusted certificate authority vouches that the key belongs to that site
Why this is the answer
Browsers trust certificates signed, directly or through a chain, by CAs in their trust store. A self-signed certificate could be made by anyone, such as a man-in-the-middle.
Answer
C · No trusted certificate authority vouches that the key belongs to that site
5.Number systems
Answer: U = 148
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
108 = 0110 1100
inverted: 1001 0011
+1 → 1001 0100
Read it as unsigned
1001 0100₂ = 256 − 108 = 148
Answer
U = 148
6.Number systems
Answer: R = 208
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 left by 3 moves every bit 3 places to the left; bits that fall off the left end are lost and zeros come in on the right.
1001 1010 << 3
= 1101 0000
Value of the result
1101 0000₂ = 208
Answer
R = 208
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