Tag: ICND2

Network Address Translation allows private IPv4 addresses to be translated to an IPv4 public address to communicate over the Internet. This saves public IP addresses as not every device needs a public IP address and if we allowed that we would’ve run out of IPv4 addresses by now.

NAT allows multiple private IPv4 addresses to be translated to one public IP address, therefore saving public IP addresses.

There are three different types of NAT.

1. Static NAT – This is a one-to-one translation, one private IP address will be translated to one public IP address
2. Dynamic NAT – You don’t need to manually map each IP static with dynamic NAT but you do need to ensure you have enough public IPs for the private IPs to be translated to.
3. Overloading (PAT) – This is a many-to-one translation, multiple private IP addresses will be translated to one public IP, kinda cool right? This is the reason we haven’t run out of IPv4 addresses yet.

Some tips when troubleshooting ACL:

• Ensure correct IP and wildcast masks are correctly entered into the ACL
• Ensure an access-group is applied to an interface
• If no traffic is permitted, all traffic will be denied, there is an explicit deny.
• Access-lists are read top to bottom, if a first match is found it will stop reading. So if a deny is specified it may block a permit statement. Order of ACL is important.
• Remarks can be added to ACL to make reading them in future easier using the access-list <number> remark “This ACL blocks FTP”

The following commands can be used on Router to verify and monitor ACL’s :

• show access-list
• show access-list 101
• show ip access-list
• show ip interfaces
• show running-config

It is possible to verify connectivity depending on the ACL by using applications such as FTP, Telnet and SSH from an end device such as a PC/Laptop.

Describe the purpose and types of access control lists – Access lists are used to restrict or allow access to traffic via an interface. You may want to allow redirect port 25 (SMTP) to your Exchange server and allow traffic to flow in and out of the port. This is possible with access lists.

There are two types of access lists standard and extended.

Standard

Standard access lists filter based on source IP address. Telnet, Web, SMTP etc cannot be filtered by a standard access list. Standard access-lists can be created between 1-99 and 1300-1999. An example:

configure terminal
access-list 1 permit 10.0.0.1 0.0.0.255

Extended

Extended access list can filter by source, destination and port. Extended access lists can be created byween 100-199 and 2000-2699.

configure terminal
access-list 100 permit tcp any eq 25 host 10.0.0.1

Both standard and extended access lists can be placed either inbound or outbound on an interface allowing for greater control on what packets can be sent/received.

configure terminal
interface s0/0
ip access-group 100 in
ip access-group 100 out

Basic router security includes but not limited to:

• Configuring an enable secret

configure terminal
enable secret 'password'

• Secure passwords

configure terminal
service password-encryption

• Displaying a motd

configure terminal
banner #motd#
banner #login#

• Locking down the console port

configure terminal
line console 0
password 'password'
login

• Configuring terminal lines for remote access

configure terminal
line vty 0 4
password 'password'
login
transport input telnet ssh (remove telnet for ssh only)