Networking Fundamentals
Layers you must be able to place
Junos questions lean on general networking. Know the seven OSI layers and what lives at each: Ethernet frames and MAC addresses at Layer 2, IP addressing, ARP and ICMP at Layer 3, TCP/UDP ports at Layer 4. A switch forwards frames by MAC address inside a broadcast domain; a router forwards packets by IP address between subnets and decrements TTL at each hop.
TCP, UDP and ports
TCP is connection oriented: a three-way handshake (SYN, SYN-ACK, ACK), retransmission, and windowing. UDP is connectionless and carries DNS queries, DHCP, TFTP, SNMP traps and most voice/video. Ports worth memorizing: SSH 22, Telnet 23, DNS 53 (both TCP and UDP), DHCP 67/68, TFTP 69, HTTP 80, NTP 123, SNMP 161/162, HTTPS 443, BGP 179, syslog 514.
Addressing and subnetting
Private ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16; 127.0.0.0/8 is loopback; 169.254.0.0/16 is link-local. A host uses ARP to learn the MAC of its default gateway and DNS to resolve names. A /n prefix holds 2^(32-n) addresses and 2^(32-n) - 2 usable host addresses.
Traps
Questions blur broadcast and collision domains (each switch port is its own collision domain), swap the roles of ARP and DNS, or offer a subnet mask one bit off. Watch for "how many usable hosts" versus "how many addresses."
Worked example
172.16.10.0/26: mask 255.255.255.192, block size 256 - 192 = 64, so the subnets are .0, .64, .128 and .192, each with 62 usable hosts. The broadcast address of the first subnet is 172.16.10.63. Rule of thumb: magic number = 256 minus the interesting mask octet.