Network Fundamentals¶

OSI Model¶

The OSI (Open Systems Interconnection) model has 7 layers:

Layer	Name	Function	Protocols/Devices
7	Application	User interface, applications	HTTP, FTP, SMTP, DNS
6	Presentation	Data format, encryption	SSL/TLS, JPEG, ASCII
5	Session	Session management	NetBIOS, RPC
4	Transport	End-to-end connections	TCP, UDP
3	Network	Routing, IP addressing	IP, ICMP, Routers
2	Data Link	Frame switching, MAC	Ethernet, Switches, ARP
1	Physical	Physical transmission	Cables, Hubs, Signals

Mnemonic: "Please Do Not Throw Sausage Pizza Away"

TCP/IP Model (Simplified)¶

Layer	OSI Equivalent	Protocols
Application	5-7	HTTP, DNS, SSH
Transport	4	TCP, UDP
Internet	3	IP, ICMP
Link	1-2	Ethernet, WiFi

Routing vs Switching¶

Layer 2 - Switching¶

Switches operate at the Data Link layer using MAC addresses.

Characteristics:

Forwards based on MAC address
Single broadcast domain (can be segmented with VLANs)
Low latency
No IP knowledge needed

# View MAC table
show mac address-table    # Cisco
bridge fdb show           # Linux

Layer 3 - Routing¶

Routers operate at the Network layer using IP addresses.

Characteristics:

Forwards based on IP address
Separate broadcast domains
Makes forwarding decisions
Connects different networks

# View routing table
route -n
ip route show
netstat -rn

Routing Table Example:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG    100    0        0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U     100    0        0 eth0

IP Addressing¶

IPv4¶

32-bit addresses, written as four octets.

192.168.1.1 = 11000000.10101000.00000001.00000001

Address Classes (Historical):

Class A: 0.0.0.0 - 127.255.255.255 (/8)
Class B: 128.0.0.0 - 191.255.255.255 (/16)
Class C: 192.0.0.0 - 223.255.255.255 (/24)

Private Ranges (RFC 1918):

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16

IPv6¶

128-bit addresses, written as eight groups of hexadecimal.

2001:0db8:85a3:0000:0000:8a2e:0370:7334
# Can be shortened:
2001:db8:85a3::8a2e:370:7334

Special Addresses:

::1 - Loopback
:: - All zeros
fe80::/10 - Link-local
ff00::/8 - Multicast

CIDR Notation¶

Classless Inter-Domain Routing specifies networks using prefix length.

Format¶

192.168.1.0/24
│           │
IP Address  Prefix length (network bits)

Subnet Calculations¶

Example: 192.168.1.0/24

Network bits: 24
Host bits: 32 - 24 = 8
Subnet mask: 255.255.255.0
Number of hosts: 2^8 - 2 = 254
Network address: 192.168.1.0
Broadcast: 192.168.1.255
Usable range: 192.168.1.1 - 192.168.1.254

Common Subnets:

CIDR	Subnet Mask	Hosts	Notes
/32	255.255.255.255	1	Single host
/31	255.255.255.254	2	Point-to-point
/30	255.255.255.252	2	Point-to-point (4-2)
/29	255.255.255.248	6	8-2
/28	255.255.255.240	14	16-2
/24	255.255.255.0	254	Class C
/16	255.255.0.0	65534	Class B
/8	255.0.0.0	16777214	Class A

Subnetting Example¶

Problem: Divide 192.168.1.0/24 into 4 subnets.

Solution:

Need 2 more bits: /24 → /26
Each subnet: 2^6 = 64 addresses

Subnets:

192.168.1.0/26 (0-63)
192.168.1.64/26 (64-127)
192.168.1.128/26 (128-191)
192.168.1.192/26 (192-255)

VLANs (Virtual LANs)¶

VLANs segment a physical network into multiple logical networks.

802.1Q Tagging¶

Adds 4-byte tag to Ethernet frame.

VLAN Tag Fields:

TPID (2 bytes): 0x8100 (identifies 802.1Q frame)
TCI (2 bytes):
PCP (3 bits): Priority
DEI (1 bit): Drop eligible
VID (12 bits): VLAN ID (1-4094)

Configuration¶

# Create VLAN interface (Linux)
ip link add link eth0 name eth0.10 type vlan id 10
ip addr add 192.168.10.1/24 dev eth0.10
ip link set eth0.10 up

# View VLANs
cat /proc/net/vlan/config

Use Cases¶

Network segmentation
Security isolation
Broadcast domain reduction
Multi-tenancy

NAT (Network Address Translation)¶

NAT translates private IP addresses to public addresses.

Types¶

SNAT (Source NAT)

Changes source address
Used for outgoing traffic
Typical home router

DNAT (Destination NAT)

Changes destination address
Port forwarding
Load balancing

PAT (Port Address Translation)

Also called NAT overload
Many private IPs → one public IP
Uses different source ports

Example¶

Internal: 192.168.1.10:5000 → External Server

NAT Gateway: 203.0.113.5:34567 → External Server

NAT Table:

Internal              External
192.168.1.10:5000 ↔  203.0.113.5:34567

# Configure NAT (Linux iptables)
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

# Port forwarding
iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination 192.168.1.10:8080

Network Namespaces¶

Linux network namespaces provide isolated network stacks.

# Create namespace
ip netns add myns

# Execute in namespace
ip netns exec myns ip addr

# Create veth pair
ip link add veth0 type veth peer name veth1

# Move one end to namespace
ip link set veth1 netns myns

# Configure
ip addr add 10.0.0.1/24 dev veth0
ip link set veth0 up

ip netns exec myns ip addr add 10.0.0.2/24 dev veth1
ip netns exec myns ip link set veth1 up

# Test
ping 10.0.0.2

Use Cases:

Container networking (Docker, Kubernetes)
Network isolation
Testing
VPNs

Practice Questions¶

Explain the difference between Layer 2 and Layer 3 devices.
How many usable hosts in a /27 subnet?
What is CIDR notation and why was it introduced?
Explain how VLANs work.
What is the difference between SNAT and DNAT?
How does a switch learn MAC addresses?
Calculate the subnet mask for /19.