Designing a Functional TCP/IP Solution

Effective TCP/IP network design requires careful consideration of addressing strategies, routing requirements, and scalability needs. This lesson examines the fundamental elements that form the foundation of any TCP/IP implementation, with particular emphasis on IP addressing methodologies and their impact on network architecture.

Four essential elements must be addressed when designing a functional TCP/IP network:

1. IP Address and Subnet Mask Configuration
   • Determine addressing scheme (classful, VLSM, or CIDR)
   • Calculate appropriate subnet masks for each network segment
   • Plan address allocation to accommodate growth
   • Balance between address conservation and operational simplicity


2. Private Network Addressing Structure
   • RFC 1918 private address ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
   • NAT (Network Address Translation) implementation strategy
   • Public IP allocation for Internet-facing services
   • IPv4/IPv6 dual-stack considerations


3. Subnet Routing Architecture
   • Routing protocol selection (static, RIP, OSPF, EIGRP, BGP)
   • Route summarization and aggregation strategies
   • Inter-VLAN routing requirements
   • Redundancy and failover mechanisms


4. Consistent Design Methodology
   • Standardized subnet sizing conventions
   • Hierarchical addressing structure
   • Documentation and IP address management (IPAM)
   • Change management procedures

Every device communicating via TCP/IP requires a unique IP address. IPv4, the predominant addressing standard, uses 32-bit addresses typically represented in dotted-decimal notation (e.g., 192.168.1.100).

IP Address Components:

• Network ID: Identifies the network or subnet to which the device belongs
• Host ID: Identifies the specific device within that network
• Subnet Mask: Determines which portion of the IP address represents the network vs. host

Address Structure: The subnet mask defines the boundary between network and host portions. For example:

• IP Address: 192.168.10.50
• Subnet Mask: 255.255.255.0 (or /24 in CIDR notation)
• Network ID: 192.168.10.0
• Host ID: 50
• Usable addresses: 192.168.10.1 - 192.168.10.254 (254 hosts)

Three primary addressing approaches exist, each with distinct characteristics and routing protocol requirements:

The original IP addressing scheme divided the address space into five classes (A, B, C, D, E) with fixed subnet masks.

Classful Address Ranges and Default Masks:

Class	Range	Default Mask	CIDR Notation	Networks / Hosts
Class A	1.0.0.0 - 126.255.255.255	255.0.0.0	/8	126 networks, 16.7M hosts each
Class B	128.0.0.0 - 191.255.255.255	255.255.0.0	/16	16,384 networks, 65,534 hosts each
Class C	192.0.0.0 - 223.255.255.255	255.255.255.0	/24	2M networks, 254 hosts each

Classful Limitations:

• Inflexible - cannot adjust mask to actual needs
• Wasteful - often allocates far more addresses than needed
• Example: Organization needs 500 hosts, must use Class B (65,534 addresses = 65,034 wasted)
• Routing protocol requirement: RIPv1 (does not advertise subnet masks)

Current Status: Classful addressing is obsolete for new deployments. Modern networks use VLSM or CIDR for efficient address utilization.

CIDR (Classless Inter-Domain Routing) notation provides a compact way to represent IP addresses and their subnet masks using a slash followed by the number of network bits.

Common CIDR Prefix Lengths:

CIDR	Subnet Mask	Usable Hosts	Common Use
/30	255.255.255.252	2	Point-to-point links
/29	255.255.255.248	6	Small server segments
/28	255.255.255.240	14	Small office networks
/27	255.255.255.224	30	Department networks
/26	255.255.255.192	62	Medium departments
/24	255.255.255.0	254	Standard subnet
/23	255.255.254.0	510	Large departments
/22	255.255.252.0	1,022	Large facilities

CIDR Benefits:

• Flexibility: Match subnet size precisely to requirements
• Efficiency: Minimal address waste through right-sizing
• Route Aggregation: Combine multiple networks into single routing table entry
• Hierarchical Design: Supports logical network organization

VLSM allows using different subnet masks within the same major network, enabling optimal address allocation for subnets of varying sizes.

VLSM Example Scenario:
Organization has 172.16.0.0/16 network allocation and needs:

• 3 department networks: 500 hosts each
• 10 small office networks: 25 hosts each
• 20 point-to-point WAN links: 2 hosts each

VLSM Allocation:

• Departments: 172.16.0.0/23, 172.16.2.0/23, 172.16.4.0/23 (510 hosts each)
• Small offices: 172.16.10.0/27, 172.16.10.32/27, ... (30 hosts each)
• WAN links: 172.16.20.0/30, 172.16.20.4/30, ... (2 hosts each)

Result: Efficient use of address space with minimal waste. Classful addressing would have been impossible to implement efficiently.

Windows administrators configure TCP/IP settings through the Network and Sharing Center or PowerShell.

GUI Configuration Steps:

1. Open Network Connections (Win+R, type ncpa.cpl)
2. Right-click network adapter → Properties
3. Select Internet Protocol Version 4 (TCP/IPv4) → Properties
4. Choose "Use the following IP address"
5. Enter IP address (e.g., 192.168.10.50)
6. Enter subnet mask (e.g., 255.255.255.0)
7. Enter default gateway (e.g., 192.168.10.1)
8. Enter DNS servers (preferred and alternate)
9. Click OK to apply configuration

PowerShell Configuration:

# Set static IP configuration
New-NetIPAddress -InterfaceAlias "Ethernet" `
    -IPAddress 192.168.10.50 `
    -PrefixLength 24 `
    -DefaultGateway 192.168.10.1

# Set DNS servers
Set-DnsClientServerAddress -InterfaceAlias "Ethernet" `
    -ServerAddresses ("8.8.8.8","8.8.4.4")

# Verify configuration
Get-NetIPAddress -InterfaceAlias "Ethernet" | Format-Table

Validation Commands:

• ipconfig /all - Display complete TCP/IP configuration
• Get-NetIPConfiguration - PowerShell equivalent
• ping [gateway] - Test local connectivity
• ping 8.8.8.8 - Test Internet connectivity
• nslookup www.example.com - Test DNS resolution

Critical Rules:

• Cannot Reduce Network Bits: When using class-based addresses or VLSM, you cannot decrease the network ID bits below the default class mask
  • Example: Class B network (172.16.0.0/16) cannot use /15 mask
  • You can only subnet further (e.g., /17, /18, /24) not broaden
• Contiguous Bits: Subnet mask must be contiguous 1s followed by contiguous 0s
  • Valid: 255.255.255.0 (11111111.11111111.11111111.00000000)
  • Invalid: 255.255.0.255 (non-contiguous - not supported)
• All-Zeros and All-Ones: Modern networks can use subnet zero and all-ones subnet (RFC 1812)
  • Historic restriction: first and last subnets were reserved
  • Current: All subnets usable (enabled by default in modern equipment)

Understanding these foundational RFCs provides deeper insight into IP addressing standards:

RFC	Title	Description
RFC 950	Internet Standard Subnetting Procedure	Original specification for IP address subnetting
RFC 1518	An Architecture for IP Address Allocation with CIDR	Defines architecture required to support CIDR
RFC 1519	Classless Inter-Domain Routing (CIDR)	CIDR addressing and route aggregation strategies
RFC 1812	Requirements for IPv4 Routers (Section 4.2.2.11)	All-zeros and all-ones subnet usage in address masks
RFC 1878	Variable Length Subnet Table for IPv4	Comprehensive VLSM tables and calculations

Additional Key RFCs:

• RFC 1918: Address Allocation for Private Internets (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
• RFC 4632: Classless Inter-domain Routing (CIDR) - most recent update

The following lesson explores private IP addressing schemes and Network Address Translation (NAT), building on the addressing fundamentals covered here. Understanding these core concepts is essential for designing scalable, efficient TCP/IP networks.