[Day 10] 7. ACL for Telnetting

Access Control Lists (ACLs) are used to permit or deny traffic based on IP addresses, protocols, or port numbers. In the case of Telnet, which uses port 23, an ACL can control which devices are allowed to initiate a Telnet session.

Let's say you have two networks:

• Network A: 172.16.10.0/24 (with a PC at 172.16.10.10)
• Network B: 172.16.30.0/24 (with a PC at 172.16.30.10)

If you want to configure an ACL that only allows the PC in Network A (172.16.10.10) to Telnet to the router, while denying Telnet access from Network B (172.16.30.0), you would do the following on the router:

Example ACL Configuration:

1. Create an ACL to Permit Telnet from Network A (PC at 172.16.10.10):
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   Router(config)# access-list 10 permit host 172.16.10.10

2. Deny Telnet from Network B (172.16.30.0):
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   Router(config)# access-list 10 deny 172.16.30.0 0.0.0.255

3. Apply the ACL to the vty lines (used for Telnet access):
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   Router(config)# line vty 0 4

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   Router(config-line)# access-class 10 in

Explanation:

• ACL 10: Specifies which IPs can Telnet to the router. It permits Telnet from 172.16.10.10 and denies it from 172.16.30.0/24.
• The access-class command applies the ACL to Telnet (on the vty lines), controlling who can access the router using Telnet.

With this configuration, only the PC at 172.16.10.10 (Network A) will be able to Telnet to the router, while any devices in Network B will be blocked.