TCP/IP networks

IPv4

commonly used (except for phones)
32-bit addresses: Class A 8 network bits, 24 host bits (1st bit = 0); Class B 16 network bits, 16 host bits (1st bits = 10); Class C 24 network bits, 8 host bits (1st bits = 110); subnet mask all network bits = 1, all host bits = 0
private addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are not routed to the public Internet
local addresses (127.0.0.0/8) are not sent off the local host
network address has all host bits = 0
broadcast address has all host bits = 1
address are running out and address assignment produces inefficient routing

organization

protocols are loosely organized into 4 layers

the layers do not exactly correspond to the 7 more rigorously defined OSI layers

Application: data streams to/from programs using network communication; DHCP provides automatic addressing (not needed with IPv6 SLAAC); DNS provides a mapping between names and addresses
Transport: end-to-end communication between programs using software ports; UDP is stateless, has low overhead; TCP is stateful, has error correction and more overhead; QUIC is intended to replace TCP with encryption and multiplexed UDP streams for better performance
Internet (Layer 3): moves packets between networks using IP addresses; IP makes a best effort to route packets to the destination network; ICMP moves messages about the network itself; NAT translates source and/or destination addresses, usually public ⇄ private (not needed with IPv6); CIDR allows any number of IP address host bits (not needed with IPv6)
Link (Layer 2): moves frames between devices on the same network using MAC addresses; ARP resolves IP addresses to MAC addresses

physical (Layer 1): interfaces, cables, etc; ethernet (IEEE 802.3); Wi-Fi (IEEE 802.11)
switch (Layer 2): link layer, moves data between hosts on a local network using MAC addresses; allows simultaneous traffic between different pairs of hosts
router (Layer 3): internet layer, moves data between hosts on a different networks using IP addresses; communicates with interconnected routers to determine the best route
firewall: filters traffic based on interfaces, direction, addresses, ports, data, etc; used to direct traffic and to block potentially malicious traffic