Prefix length

2.1 Explain characteristics of routing technologies

Route Selection

📘CompTIA Network+ (N10-009)

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Prefix Length in Route Selection

Prefix length is a critical concept in networking that helps routers determine the most specific route to send traffic. It’s closely tied to subnetting and IP addressing.

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1. What is Prefix Length?

• The prefix length tells you how many bits of an IP address are used to identify the network portion.
• It’s written in CIDR (Classless Inter-Domain Routing) notation, for example:
  • 192.168.1.0/24
  • /24 is the prefix length
• The bigger the number, the more specific the network:
  • /24 → 255.255.255.0 → 256 addresses
  • /28 → 255.255.255.240 → 16 addresses

Key Idea:

• The network with the longest prefix (most bits in the network portion) is the most specific route.
• Routers use this to make routing decisions when multiple routes exist to the same destination.

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2. Why Prefix Length Matters in Route Selection

When a router receives traffic, it looks at its routing table to decide where to send the packet. If there are multiple matching routes, the router chooses:

1. The route with the longest prefix length (most specific)
2. If the prefix lengths are equal, the router may use administrative distance to decide

Example Routing Table:

Destination	Prefix	Next Hop
192.168.1.0/24	/24	Router A
192.168.1.0/16	/16	Router B

• Traffic for 192.168.1.50 → matches both routes
• Which route is used?
  • /24 is more specific than /16, so Router A is chosen.

Tip for the exam: Longest prefix = most specific = preferred route.

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3. How Prefix Length Works with Subnets

• Prefix length defines the size of the subnet.
• A smaller prefix length → bigger subnet → more IP addresses
• A larger prefix length → smaller subnet → fewer IP addresses, more specific routing

Example:

Network	Prefix Length	Number of IPs	Specificity
10.0.0.0/8	/8	16,777,216	Low
10.1.0.0/16	/16	65,536	Medium
10.1.1.0/24	/24	256	High

Routing Decision:

• Packet for 10.1.1.25 → sent to /24 route, because it’s the most specific.

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4. How This Appears in IT Environments

• Internal Networks:
  • Large company networks often have multiple subnets. Routers use prefix length to make sure traffic goes to the correct department or VLAN.
• Internet Routing:
  • ISPs advertise networks with different prefix lengths.
  • Routers always prefer the longest prefix match to direct traffic accurately.
• Firewall & ACL Rules:
  • When applying rules, network admins often use prefix lengths to target specific subnets.
• VPNs:
  • When multiple VPN routes exist, routers use prefix length to decide which VPN tunnel to send traffic through.

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5. Key Points to Remember for the Exam

1. Prefix length = number of network bits in the IP address.
2. Longest prefix match is always preferred for route selection.
3. Smaller subnets (larger prefix lengths) = more specific route.
4. If multiple routes have the same prefix length, routers fall back on administrative distance.
5. Found in routing tables, used in both IPv4 and IPv6 networks.

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6. Quick Memory Trick

• Think: “More bits = more specific = preferred route”
• Example: /28 > /24 > /16 > /8 when deciding which route to take.

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✅ Exam Tip:
On CompTIA Network+, you may get a routing table and a destination IP. Your task will be to identify which route the router will choose, and the rule is always: longest prefix match wins.