Internet Service Providers: How ISPs Assign IP Addresses

Internet Service Providers (ISPs) are the gateway to the internet, responsible for assigning IP addresses to customers and routing traffic. Understanding how ISPs work, how they assign IP addresses, and the different types of connections helps you make informed decisions about internet service. This comprehensive guide explains everything you need to know about ISPs and IP address assignment.

What is an ISP?

An Internet Service Provider (ISP) is a company that provides internet access to customers. ISPs connect your home or business to the global internet, assign IP addresses, and route your traffic to its destination using BGP.

ISP Hierarchy

Tier 1 ISPs:

Global backbone providers
Own international infrastructure
Peer with other Tier 1s
No upstream providers
Examples: AT&T, Verizon, Level 3, NTT

Tier 2 ISPs:

Regional providers
Buy transit from Tier 1
Peer with some Tier 2s
Sell to Tier 3 and customers
Examples: Regional carriers

Tier 3 ISPs:

Local providers
Buy transit from Tier 2
Serve end customers
No peering agreements
Examples: Local ISPs, cable companies

Network path:

Your Device → Tier 3 ISP → Tier 2 ISP → Tier 1 ISP → Internet

How ISPs Get IP Addresses

Regional Internet Registries (RIRs)

Five RIRs worldwide:

ARIN (American Registry for Internet Numbers):

Region: North America
Countries: USA, Canada, Caribbean
IPv4: Exhausted (2015)
IPv6: Available

RIPE NCC (Réseaux IP Européens):

Region: Europe, Middle East, Central Asia
Countries: 75+ countries
IPv4: Exhausted (2012)
IPv6: Available

APNIC (Asia-Pacific Network Information Centre):

Region: Asia-Pacific
Countries: 56+ countries
IPv4: Exhausted (2011)
IPv6: Available

LACNIC (Latin America and Caribbean Network Information Centre):

Region: Latin America, Caribbean
Countries: 33+ countries
IPv4: Exhausted (2014)
IPv6: Available

AFRINIC (African Network Information Centre):

Region: Africa
Countries: 54+ countries
IPv4: Limited remaining
IPv6: Available

IP Address Allocation Process

ISP requests addresses:

1. ISP applies to RIR
2. Justifies need (customer base, growth)
3. RIR allocates IP block
4. ISP receives /20, /19, /18, etc.
5. ISP assigns to customers

Example allocation:

ISP receives: 203.0.113.0/20
Total addresses: 4,096
Can assign to: ~4,000 customers
Subnetting: Various sizes

IPv4 Exhaustion Impact

Current situation:

All RIRs exhausted or nearly exhausted
No new IPv4 allocations
ISPs must:
- Use existing allocations efficiently
- Buy addresses on secondary market
- Implement CGNAT
- Deploy IPv6

IPv4 market prices:

2014: $5-10 per address
2024: $50-60 per address
Trend: Continuing to rise

Types of IP Address Assignments

Dynamic IP Addresses

How it works:

ISP assigns IP from pool
Changes periodically
Lease-based (DHCP)
Most common for residential

Assignment process:

1. Modem connects to ISP
2. Requests IP via DHCP
3. ISP assigns from available pool
4. Lease duration: hours to days
5. Renewal or new IP on reconnect

Characteristics:

Changes on reconnect
Shared pool among customers
Cost-effective for ISP
Sufficient for most users

When IP changes:

Modem restart
Power outage
Lease expiration
ISP maintenance
Network changes

Advantages:

Lower cost
No setup needed
Automatic configuration
Sufficient for browsing

Disadvantages:

Can't host servers easily
IP changes periodically
Port forwarding complex
Remote access difficult

Static IP Addresses

How it works:

ISP assigns permanent IP
Never changes
Dedicated to customer
Additional cost

Assignment:

Customer requests static IP
ISP provisions from reserved pool
Configured on modem/router
Remains constant

Characteristics:

Permanent assignment
Dedicated to customer
Higher cost ($5-20/month)
Business-oriented

Advantages:

Consistent address
Easy remote access
Host servers
Better for VPN
Professional services

Disadvantages:

Additional cost
Easier to track
Target for attacks
Requires security

Use cases:

Web servers
Email servers
VPN endpoints
Remote access
Business services
Security cameras

DHCP Lease Times

Typical ISP lease durations:

Cable ISPs:

Lease: 7-30 days
Renewal: Automatic
Change: Rare if modem stays on

DSL ISPs:

Lease: 24 hours - 7 days
Renewal: On reconnect
Change: More frequent

Fiber ISPs:

Lease: 24 hours - indefinite
Renewal: Varies
Change: Depends on ISP

Mobile carriers:

Lease: Per session
Renewal: Each connection
Change: Every reconnect

IP Assignment Methods

PPPoE (Point-to-Point Protocol over Ethernet)

Common with: DSL, some fiber

How it works:

1. Modem establishes PPPoE session
2. Authenticates with username/password
3. ISP assigns IP address
4. Session maintained

Configuration:

Router: PPPoE mode
Username: customer@isp.com
Password: [provided by ISP]
IP: Assigned dynamically

Characteristics:

Authentication required
Session-based
IP assigned per session
Common in DSL

DHCP (Dynamic Host Configuration Protocol)

Common with: Cable, fiber

How it works:

1. Modem sends DHCP Discover
2. ISP DHCP server responds
3. Modem requests offered IP
4. ISP confirms assignment

Process:

Modem MAC → ISP DHCP server
Server assigns IP from pool
Lease time specified
Automatic renewal

Characteristics:

Automatic configuration
No authentication at IP level
Lease-based
Most common method

Static Assignment

Common with: Business services

How it works:

ISP provisions specific IP
Configured on modem/router
Manual or automatic setup
Permanent assignment

Configuration:

IP: 203.0.113.45
Subnet: 255.255.255.248 (/29)
Gateway: 203.0.113.46
DNS: ISP-provided

IPv6 Assignment by ISPs

Prefix Delegation

How ISPs assign IPv6:

Customer gets IPv6 prefix (not single address)
Typical: /56 or /48
Contains thousands of subnets
Each subnet: /64

Example:

ISP assigns: 2001:db8:1234::/48
Customer can create: 65,536 /64 subnets
Each subnet: 18 quintillion addresses

DHCPv6 Prefix Delegation:

1. Router requests prefix
2. ISP delegates /48 or /56
3. Router assigns /64 to LANs
4. Devices auto-configure (SLAAC)

Dual-Stack Deployment

Most ISPs provide:

IPv4: Single dynamic IP (or CGNAT)
IPv6: /56 or /48 prefix
Both: Simultaneous operation

Benefits:

Access all content
Future-proof
Better performance (IPv6)
Backward compatible (IPv4)

Carrier-Grade NAT (CGNAT)

Why ISPs Use CGNAT

IPv4 exhaustion solution:

Share one public IP among many customers
Conserve IPv4 addresses
Delay IPv6 transition
Cost-effective

How it works:

Customer → Private IP (100.64.x.x)
ISP CGNAT → Public IP (shared)
Internet ← Shared public IP

Impact on customers:

Port forwarding impossible
Gaming NAT: Strict
VoIP issues
P2P problems
Can't host servers

Detection:

Router WAN IP: 100.64.x.x
Public IP (check online): Different
Result: Behind CGNAT

Solutions:

Request public IP (may cost extra)
Use IPv6
VPN with port forwarding
Cloud services

ISP Connection Types

Cable Internet

Technology: DOCSIS over coaxial cable

IP assignment:

Method: DHCP
Type: Dynamic (usually)
Lease: 7-30 days
Shared: Neighborhood bandwidth

Characteristics:

Speed: 100 Mbps - 1 Gbps
Latency: 15-30ms
Availability: Urban/suburban
Shared bandwidth

Providers: Comcast, Spectrum, Cox

DSL (Digital Subscriber Line)

Technology: Phone line

IP assignment:

Method: PPPoE or DHCP
Type: Dynamic
Lease: 24 hours - 7 days
Dedicated: Line per customer

Characteristics:

Speed: 1-100 Mbps
Latency: 20-50ms
Availability: Widespread
Distance-dependent speed

Providers: AT&T, CenturyLink, Frontier

Fiber (FTTH/FTTP)

Technology: Fiber optic cable

IP assignment:

Method: DHCP or static
Type: Dynamic or static
Lease: Varies
Dedicated: Fiber per customer

Characteristics:

Speed: 100 Mbps - 10 Gbps
Latency: 1-10ms
Availability: Growing
Symmetric speeds

Providers: Verizon Fios, Google Fiber, AT&T Fiber

Mobile/Cellular

Technology: 4G LTE, 5G

IP assignment:

Method: Per session
Type: Dynamic
Lease: Per connection
CGNAT: Common

Characteristics:

Speed: 10-1000 Mbps
Latency: 20-100ms
Availability: Widespread
Variable performance

Providers: Verizon, T-Mobile, AT&T

Satellite

Technology: Satellite communication

IP assignment:

Method: DHCP
Type: Dynamic
Lease: Varies
CGNAT: Common

Characteristics:

Speed: 25-150 Mbps
Latency: 500-700ms (high)
Availability: Rural/remote
Weather-dependent

Providers: Starlink, HughesNet, Viasat

ISP IP Address Management

Address Pool Management

Dynamic pool:

Total addresses: 10,000
Active customers: 8,000
Oversubscription: 1.25:1
Assumption: Not all online simultaneously

Pool allocation:

Residential dynamic: 80%
Business static: 10%
Infrastructure: 5%
Reserved: 5%

Geographic Assignment

IP blocks by region:

ISPs often have regional blocks
Geolocation databases track
Used for:
- Content delivery
- Advertising
- Access control
- Fraud prevention

Example:

ISP: Comcast
Region: San Francisco
Block: 76.102.0.0/16
Geolocation: San Francisco, CA

Changing ISPs and IP Addresses

What Happens When You Switch ISPs

Old ISP:

IP address returned to pool
No longer assigned to you
Reassigned to another customer

New ISP:

Different IP address
Different IP block
Different geolocation
New network path

Impact:

New public IP
Services need updating
Port forwards reconfigure
DNS propagation time

Keeping Same IP

Generally not possible:

Different ISPs = Different IP blocks
IP belongs to ISP, not customer
Exception: Business services (rare)

Workarounds:

Dynamic DNS
VPN with static IP
Cloud services
Accept IP changes

ISP Responsibilities

Network Management

Traffic routing:

Route customer traffic
Maintain infrastructure
Peering agreements
Bandwidth management

Quality of Service:

Maintain uptime
Manage congestion
Prioritize traffic
Monitor performance

Security

DDoS protection:

Detect attacks
Mitigate threats
Protect infrastructure
Customer protection

Abuse handling:

Monitor for spam
Detect malware
Handle complaints
Enforce terms of service

Customer Support

Technical support:

Connection issues
Configuration help
Troubleshooting
Equipment support

Account management:

Billing
Service changes
Upgrades
Static IP requests

Choosing an ISP

Factors to Consider

Speed:

Download requirements
Upload requirements
Simultaneous users
Usage patterns

Reliability:

Uptime guarantees
Outage history
Customer reviews
Infrastructure quality

IP address options:

Static IP available?
IPv6 support?
CGNAT used?
Cost for static IP?

Cost:

Monthly fee
Installation cost
Equipment rental
Static IP fee
Contract terms

Support:

Support hours
Response time
Technical expertise
Customer satisfaction

Conclusion

ISPs play a crucial role in internet connectivity, managing IP address allocation and routing traffic to the global internet. Understanding how ISPs assign IP addresses, the different connection types, and assignment methods helps you make informed decisions about internet service and troubleshoot connectivity issues.

Related Articles

IP Assignment

• DHCP - ISP IP assignment
• Static vs Dynamic IP - ISP IP types
• Change IP Address - Getting new IP from ISP
• IP Lease Time - ISP lease times

ISP Technologies

• Carrier-Grade NAT - ISP NAT
• IPv4 Exhaustion - Why ISPs use CGNAT
• IPv6 Adoption - ISP IPv6 deployment
• Dedicated IP - ISP dedicated IPs

Privacy

• ISP Tracking - What ISPs see
• Hide IP Address - Privacy from ISP
• VPN Basics - Bypass ISP monitoring

Network Concepts

• BGP - ISP routing
• Routing - Internet routing

Explore More

• Networking Basics - Essential concepts

Key takeaways: - ISPs get IP blocks from RIRs - Most residential: Dynamic IP via DHCP - Business: Static IP available (extra cost) - IPv4 exhaustion led to CGNAT - IPv6: Prefix delegation (/48 or /56) - Connection type affects IP assignment - CGNAT prevents port forwarding - Static IP needed for hosting - IP changes when switching ISPs - ISPs manage traffic and security

Your ISP determines your IP address, connection quality, and available services. Understanding IP assignment methods and ISP capabilities helps you choose the right service level, troubleshoot issues, and optimize your internet connection for your specific needs.