NAT Questions

Question 1

Question 2

Explanation

First we will not mention about the effect of the “extendable” keyword. So the purpose of the command “ip nat inside source static tcp 192.168.1.50 80 209.165.201.1 8080” is to translate packets on the inside interface with a source IP address of 192.168.1.50 and port 80 to the IP address 209.165.201.1 with port 8080. This also implies that any packet received on the outside interface with a destination address of 209.165.201.1:8080 has the destination translated to 192.168.1.50:80. Therefore answer C is correct.

Answer A is not correct this command “allows host 192.168.1.50 to access external websites using TCP port 80”, not port 8080.

Answer B is not correct because it allows external clients to connect to a web server at 209.165.201.1. The IP addresses of clients should not be 209.165.201.1.

Answer D is not correct because the configuration is correct.

Now we will talk about the keyword “extendable”.

Usually, the “extendable” keyword should be added if the same Inside Local is mapped to different Inside Global Addresses (the IP address of an inside host as it appears to the outside network). An example of this case is when you have two connections to the Internet on two ISPs for redundancy. So you will need to map two Inside Global IP addresses into one inside local IP address. For example:

NAT router: ip nat inside source static 192.168.1.1 200.1.1.1 extendable ip nat inside source static 192.168.1.1 200.2.2.2 extendable //Inside Local: 192.168.1.1 ; Inside Global: 200.1.1.1 & 200.2.2.2

In this case, the traffic from ISP1 and ISP2 to the Server is straightforward as ISP1 will use 200.1.1.1 and ISP2 will use 200.2.2.2 to reach the Server. But how about the traffic from the Server to the ISPs? In other words, how does NAT router know which IP (200.1.1.1 or 200.2.2.2) it should use to send traffic to ISP1 & ISP2 (this is called “ambiguous from the inside”). We tested in GNS3 and it worked correctly! So we guess the NAT router compared the Inside Global addresses with all of IP addresses of the “ip nat outside” interfaces and chose the most suitable one to forward traffic.

This is what Cisco explained about “extendable” keyword:

“They might also want to define static mappings for a particular host using each provider’s address space. The software does not allow two static translations with the same local address, though, because it is ambiguous from the inside. The router will accept these static translations and resolve the ambiguity by creating full translations (all addresses and ports) if the static translations are marked as “extendable”. For a new outside-to-inside flow, the appropriate static entry will act as a template for a full translation. For a new inside-to-outside flow, the dynamic route-map rules will be used to create a full translation”.

(Reference: http://www.cisco.com/en/US/technologies/tk648/tk361/tk438/technologies_white_paper09186a0080091cb9.html)

But it is unclear, what will happen if we don’t use a route-map?

Question 3

Explanation

The command “ip nat inside source list 1 int s0/0 overload” translates all source addresses that pass access list 1, which means all the IP addresses, into an address assigned to S0/0 interface. Overload keyword allows to map multiple IP addresses to a single registered IP address (many-to-one) by using different ports.

Question 4

Explanation

The command “ip nat inside source list 10 interface FastEthernet0/1 overload” configures NAT to overload on the address that is assigned to the Fa0/1 interface.

Question 5

Explanation

This is a static NAT command which translates all the packets received in the inside interface with a source IP address of 172.16.10.8:8080 to 172.16.10.8:80. The purpose of this NAT statement is to redirect TCP Traffic to Another TCP Port.

Question 6

Explanation

NAT64 provides communication between IPv6 and IPv4 hosts by using a form of network address translation (NAT). There are two different forms of NAT64, stateless and stateful:

+ Stateless NAT64: maps the IPv4 address into an IPv6 prefix. As the name implies, it keeps no state. It does not save any IP addresses since every v4 address maps to one v6 address. Stateless NAT64 does not conserve IP4 addresses.
+ Stateful NAT64 is a stateful translation mechanism for translating IPv6 addresses to IPv4 addresses, and IPv4 addresses to IPv6 addresses. Like NAT44, it is called stateful because it creates or modifies bindings or session state while performing translation (1:N translation). It supports both IPv6-initiated and IPv4-initiated communications using static or manual mappings. Stateful NAT64 converses IPv4 addresses.

Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enterprise-ipv6-solution/white_paper_c11-676278.html

Question 7

Explanation

The “ip nat allow-static-host” command enables static IP address support. Dynamic Address Resolution Protocol (ARP) learning will be disabled on this interface, and NAT will control the creation and deletion of ARP entries for the static IP host.

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_nat/configuration/12-4/nat-12-4-book/iadnat-addr-consv.html

Question 8

Explanation

Network Address Translation-Protocol Translation (NAT-PT) has been deemed deprecated by IETF because of its tight coupling with Domain Name System (DNS) and its general limitations in translation. IETF proposed NAT64 as the viable successor to NAT-PT.

NAT64 technology facilitates communication between IPv6-only and IPv4-only hosts and networks (whether in a transit, an access, or an edge network). This solution allows both enterprises and ISPs to accelerate IPv6 adoption while simultaneously handling IPv4 address depletion. All viable translation scenarios are supported by NAT64, and therefore NAT64 is becoming the most sought translation technology.

Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enterprise-ipv6-solution/white_paper_c11-676278.html

Question 9

Question 10

Explanation

The syntax should be: ipv6 nat prefix ipv6-prefix / prefix-length (for example: Router# ipv6 nat prefix 2001:DB8::/96)

Question 11

Explanation

From the link: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_nat/configuration/15-mt/nat-15-mt-book/iadnat64-stateful.pdf

Restrictions for configuring Stateful Network Address:

+ Virtual routing and forwarding (VRF)-aware NAT64 is not supported -> Answer A is correct.
+ IP Multicast is not supported -> Answer B is correct.
+ Application-level gateways (ALGs) FTP and ICMP are not supported -> Answer C is not correct.
+ Only TCP and UDP Layer 4 protocols are supported for header translation -> Answer E is not correct.
+ For Domain Name System (DNS) traffic to work, you must have a separate working installation of DNS64 -> This statement means stateful NAT64 supports DNS64 but we cannot conclude it is the only one supported by NAT64. We are not sure but maybe stateful NAT64 also supports DNS ALG.

Question 12

Explanation

NAT use four types of addresses:

* Inside local address – The IP address assigned to a host on the inside network. The address is usually not an IP address assigned by the Internet Network Information Center (InterNIC) or service provider. This address is likely to be an RFC 1918 private address.
* Inside global address – A legitimate IP address assigned by the InterNIC or service provider that represents one or more inside local IP addresses to the outside world.
* Outside local address – The IP address of an outside host as it is known to the hosts on the inside network.
* Outside global address – The IP address assigned to a host on the outside network. The owner of the host assigns this address.

Question 13

Question 14