Ruankao Network Administrator (Junior) cheat sheet

State whether addresses refer to bytes or words before calculating capacity.

Distinguish PC, instruction register, address register and data register.

For n-bit two’s complement the range is−2^(n−1) through 2^(n−1)−1.

Cache locality and pipeline hazards matter more than a clock-rate label alone.

OSI upper three layers (session, presentation, application) map to TCP/IP's single application layer

PDU order: bit → frame → packet → segment

Shannon uses S/N as a power ratio; convert dB first

Transmission delay = frame length / bandwidth; propagation delay = distance / velocity

A switch splits collision domains, not broadcast domains

802.1 Q is VLAN tagging, 802.1 X is port-based access control

Cat 5 e supports 1000 BASE-T to 100 m; Cat 6 is limited (~55 m) at 10 GBASE-T

Single-mode = laser, long haul; multimode = LED/VCSEL, short reach

Hamming: 2^r ≥ m + r + 1, check bits at positions 1,2,4,8

A suitable degree-r CRC detects all bursts of length at most r under the stated model.

QAM-16 = 4 bits/symbol, QAM-64 = 6 bits/symbol

A hub is a single collision domain; repeaters do not filter frames

Ethernet frame 64–1518 bytes; minimum payload 46 bytes after padding

802.1 Q tag is 4 bytes, VID is 12 bits, usable VLANs 1–4094

Root bridge = lowest priority then lowest MAC; root port = lowest path cost

Inter-VLAN traffic requires a Layer 3 device (sub-interfaces or SVIs)

CHAP is challenge-based and safer than PAP, which sends plaintext

RSTP (802.1 w) replaces blocking/listening with a discarding state

For ordinary IPv 4 LAN subnets use 2^h−2; /31 point-to-point and /32 routes are exceptions.

RIP: hop count, max 15, 16 = unreachable, UDP 520, 30 s updates

OSPF cost and timers follow the actual reference-bandwidth, network-type and implementation settings.

BGP runs over TCP 179 and is a path-vector inter-AS protocol

PAT multiplexes many private hosts onto one public address using ports

IPv 6 has no broadcast; NDP replaces ARP

Handshake: SYN → SYN+ACK → ACK; next seq = previous seq + data length

In the ordinary active-close path TIME_WAIT occurs at the active closer; simultaneous close and the specified TCP state path need separate analysis

DNS port 53, TCP for zone transfer; records A/AAAA/CNAME/MX/NS/PTR

DHCP DORA with client 68 and server 67; renew at 50% of lease

FTP 21 control, 20 data (active); passive mode uses a server-chosen port

SNMP 161 polling, 162 traps; syslog 514; NTP 123

An interface must be administratively enabled under the actual platform; do not assume every factory state requires the same command

Single-arm routing needs 802.1 Q encapsulation plus an IP address per sub-interface

ACLs use inverse (wildcard) masks: /24 → 0.0.0.255

Firewall policies are first-match; an implicit deny usually ends the list

CAPWAP uses UDP 5246 (control) and 5247 (data) between AP and controller

Trunk native VLAN mismatch causes forwarding anomalies

Hierarchy: access, distribution, core - each with distinct roles

Permanent link ≤ 90 m; full channel ≤ 100 m including patch cords

Six cabling subsystems: work area, horizontal, backbone, telecom room, equipment room, entrance facility

Cat 5 e ≈ 1 G, Cat 6/6 a ≈ 10 G; fibre for backbone and long runs

Acceptance testing: wire map, length, attenuation, NEXT, fibre loss, failover

Deliverables include as-built drawings, IP/VLAN plan and configuration baseline

Compute effective share and filesystem access using the actual inheritance and permission rules.

rwx = 4/2/1; chmod 755 = owner rwx, others r-x

Key paths: /etc, /var/log, /etc/named.conf, /etc/httpd/conf/httpd.conf, SSH port 22

RAID 5 usable = (n−1)×disk, tolerates one failure; RAID 10 = n/2 capacity

Type 1 hypervisor runs on bare metal; type 2 runs on a host OS

A same-store snapshot alone is not independent protection against loss of that store.

Internet-facing services belong in a DMZ; data stores sit in a filtered internal zone.

VLANs alone are not a trust boundary - add DAI, private VLANs and 802.1 X.

IPsec AH gives integrity only; ESP adds confidentiality.

IDS is passive and alerts; IPS is inline and blocks.

NAT is not a security control and temporary rules must be removed.