01 - Network Fundamentals (Domain 1, 20%)¶

OSI vs TCP/IP¶

(See fact-sheet.md for the layered table.) Memorize what protocols live at each layer.

Why OSI vs TCP/IP¶

OSI is a teaching model with 7 layers. TCP/IP is the operational model with 4 layers. Cisco still tests OSI; real networks run TCP/IP.

Most CCNA questions ask "at which layer does X operate?" Examples:

• TCP - layer 4 (Transport)
• UDP - layer 4
• IP - layer 3 (Network)
• ICMP - layer 3 (network-layer, but encapsulated in IP)
• MAC addresses, ARP, VLANs, STP - layer 2 (Data Link)
• Hubs, repeaters, cabling - layer 1 (Physical)
• Switches - layer 2 (some are layer 3 capable)
• Routers - layer 3
• Firewalls - varies (L3, L4, L7 depending on type)
• Load balancers - L4 or L7
• HTTP, DNS, DHCP, FTP - layer 7 (Application)

TCP vs UDP¶

TCP 3-way handshake¶

SYN → SYN/ACK → ACK

TCP teardown¶

FIN → ACK → FIN → ACK

IP addressing¶

IPv4 classes (legacy, but tested)¶

127.0.0.0/8 is loopback (technically class A but reserved).

Special / reserved IPv4¶

• 127.0.0.1 - loopback
• 169.254.0.0/16 - APIPA (link-local)
• 0.0.0.0 - default route / unspecified
• 255.255.255.255 - limited broadcast (not routed)

Private (RFC 1918)¶

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

CIDR (Classless Inter-Domain Routing)¶

VLSM (Variable Length Subnet Masking) lets you carve subnets of varying sizes. Mandatory on CCNA.

Subnetting (the most-tested skill)¶

The pattern¶

For a /N subnet:

• Host bits = 32 - N
• # Hosts = 2^host_bits - 2 (subtract 2 for network and broadcast)
• # Subnets in a parent network = 2^borrowed_bits
• Block size = 256 - last_octet_of_mask (for /N where 24 < N < 32)

Worked example¶

Network: 192.168.10.0/26

• /26 = 26 mask bits, 6 host bits
• Hosts = 2^6 - 2 = 62
• Block size = 256 - 192 = 64
• Subnets in 192.168.10.0/24:
• 192.168.10.0/26 (network), .1-.62 (hosts), .63 (broadcast)
• 192.168.10.64/26 (network), .65-.126, .127 (broadcast)
• 192.168.10.128/26 (network), .129-.190, .191 (broadcast)
• 192.168.10.192/26 (network), .193-.254, .255 (broadcast)

Practice¶

You should be able to instantly answer:

• "What's the network address of 10.20.30.45/27?" → block size 32 → 10.20.30.32
• "How many hosts in 172.16.0.0/22?" → 2^10 - 2 = 1022
• "What's the broadcast of 192.168.5.100/28?" → block size 16; this is in the .96 subnet → broadcast .111

If this isn't fast yet, drill it.

IPv6¶

Address structure¶

• 128 bits = 32 hex chars = 8 groups of 4
• Compress runs of zero groups with :: (only once per address)
• 2001:0db8:0000:0000:0000:0000:0000:0001 = 2001:db8::1

Address types¶

Common multicast groups¶

• ff02::1 - all nodes on segment
• ff02::2 - all routers
• ff02::5 - OSPFv3 all routers
• ff02::9 - RIPng

EUI-64¶

Auto-generates a 64-bit interface ID from a 48-bit MAC:

1. Split MAC: aabb:ccdd:eeff → aabb:cc | dd:eeff
2. Insert fffe: aabb:cc | fffe | dd:eeff
3. Flip 7^th bit (universal/local): a8bb:ccff:fedd:eeff

IPv6 address acquisition¶

• Static
• SLAAC (StateLess Address AutoConfiguration) using router advertisements
• DHCPv6 (stateful) - similar to DHCPv4
• DHCPv6-PD (prefix delegation) - get an entire prefix, common for ISP-to-customer

Wireless basics¶

Frequency bands¶

• 2.4 GHz - longer range, more interference, fewer non-overlapping channels (1, 6, 11 in NA)
• 5 GHz - shorter range, less interference, many non-overlapping channels
• 6 GHz (Wi-Fi 6E / Wi-Fi 7) - newest, even less interference

Wireless architectures¶

• Autonomous AP - standalone, manages itself
• Lightweight AP + WLC (controller) - centralized management; APs use CAPWAP to talk to WLC
• Cloud-managed - APs phone home to a SaaS controller (Cisco Meraki)

CAPWAP¶

• Control channel: UDP 5246
• Data channel: UDP 5247

Network virtualization concepts¶

CCNA touches lightly on:

• Virtual machines - hypervisors (VMware ESXi, KVM, Hyper-V)
• Containers - Docker, Kubernetes; share OS kernel; lighter than VMs
• Virtual switches - vSwitch (VMware), Open vSwitch (KVM); software-defined L2 inside hypervisor
• Cloud networking - VPCs, route tables, internet gateways. The same TCP/IP, applied to managed cloud constructs.

Cabling and topologies¶

Topology types¶

• Star - all nodes connect to a central switch (most common)
• Mesh - every node connects to every other (full mesh) or many (partial mesh)
• Ring - rare in modern enterprise
• Bus - legacy
• Hybrid - combination

Verification commands you should know¶

show version                  # IOS version, uptime, boot info
show running-config           # current config
show startup-config           # saved config
show ip interface brief       # quick interface status
show interfaces               # detailed
show interfaces gi0/1
show interfaces description
show interfaces status
show mac address-table
show arp
show cdp neighbors
show cdp neighbors detail
show lldp neighbors
ping 8.8.8.8
ping 8.8.8.8 source loopback0
traceroute 8.8.8.8