Planning the Addressing Structure

1. Planning the Addressing Structure Working at a Small-to-Medium Business or ISP – Chapter 4

2. Objectives • Describe how IP Addressing is implemented in the LAN • Subnet a given network to allow for efficient use of IP address space • Explain how Network Address Translation (NAT) and Port Address Translation (PAT) are used in a network

3. Implementation of IP Addressing in the LAN • The purpose of an IP address • IP address hierarchical structure • The classes of IP addresses

4. Review of IP addressing • IP addressing is the method used to identify hosts and network devices. • In order to send and receive messages on an IP network, every network host must be assigned a unique 32 bit IP address. • Because large binary numbers are difficult for humans to read and understand, IP addresses are usually displayed in dotted-decimal notation. • IP addresses are hierarchical therefore for a network, this means that part of the 32-bit number identifies the network (parent) while the rest of the bits identify the host (child).

5. Review of IP addressing • As the number of hosts connected to the Internet continues to grow, and the IP addressing scheme has to be adapted to cope with this growth. • In order to cope with the demand, more unique network numbers were required to create more possible network designations, the 32-bit address space was organized into five classes. • Three of these classes, A, B, and C, provide addresses that can be assigned to individual hosts or networks. The other two classes, D and E, are reserved for multicast and experimental use respectively.

6. Review of IP addressing

7. Implementation of IP Addressing in the LAN • Classful subnetting including how subnet masks are used in calculations for addressing and routing, and IP address notation for subnet masks

8. Classful Subnetting • RFC 917, Internet Subnets, defines the subnet mask as the method routers use to isolate a subnet from an IP address. • When a router receives a packet it uses the destination IP address in the packet and the subnet masks associated with the routes in its routing table to determine the appropriate path on which to forward the packet. • The router reads the subnet mask from left to right, bit for bit. • If a bit in the subnet mask is set to 1, it indicates that the value in that position is part of the network ID. • A 0 in the subnet mask indicates that the value in that position is part of the host ID.

9. Classful Subnetting • The two-level hierarchy of classed addressing included a network ID and a host ID. • In classful subnetting, the network ID is left alone, and the host ID is divided into a subnet ID and a new host ID. • For example, a Class B network has a 16-bit default subnet mask of 11111111 11111111 00000000 00000000, or 255.255.0.0. That leaves 16-bits for the host ID. • To divide a class B into multiple networks is to use four of the host bits as a subnet ID. There is now a 20-bit subnet mask of 255.255.240.0, and only 12-bits remain for the host ID.

10. Implementation of IP Addressing in the LAN • Identifing the number of subnet bits required for a given network implementation

11. Classful Subnetting • Subdividing a network adds a level to the network hierarchy. Now there are three levels: a network, a subnetwork, and a host. How are these three levels identified? • In classful addressing, the number of network bits is fixed. There are 8 bits that designate a Class A network, 16 bits for a Class B, and 24 for a Class C. That leaves the host bits as the only part of the IP address with any flexibility to modify. • There are two considerations when planning subnets: the number of hosts on each network, and the number of individual local networks needed.

12. Classless Subnetting • CIDR • VLSM

13. Classless Subnetting • Partitioning the host ID this way always results in a fixed number of subnets and a fixed number of hosts per subnet. • In a situation where an organization has a Class B network with four subnets, thousands of IP addresses can be wasted if some of the subnets have only a few hosts in them. • Therefore to use IP addresses more efficiently, Classless Inter-Domain Routing (CIDR) was created. • With CIDR, there are no more network classes. CIDR uses variable length subnet masks (VLSM) for subnetting. The network ID no longer has to be on an octet boundary.

14. Classless Subnetting • Using CIDR addressing, sometimes referred to as classless addressing, the number of bits that can make up the network ID is not restricted by class. • Networks can be created that use the 192.168.0.0 address space with fewer than 24 bits indicating the network number.

15. Creating Custom Subnet Masks • Communicating between subnets

16. Custom Subnet Masks • The number of bits for a subnet ID that will be added to the subnet mask depends on several factors. • For instance, in an organization assigned a Class C address, what if there are multiple networks, one network with 7 hosts, another with 60 hosts, and a third with 34 hosts? • In classed subnetting, all subnets must be the same size, which means that the minimum number of hosts that each subnet must support is 60. • To support a minimum number of 60 hosts, at least 6 bits are required in the host ID, which leaves 2 bits for the subnet identifier. Under these conditions, four subnets can be created, each with 64 hosts.

17. Custom Subnet Masks • If a Class C network is subnetted and 3 bits are taken from the host ID to use for the subnet ID, there are 5 bits left for host addresses. Five host bits mean that there can be 30 hosts per subnet, or 2^5 - 2. • The number of subnets is calculated in a similar manner. If 3 bits are used for the subnet address, the number of subnets is 2x2x2, or 2^3. By subnetting in this manner, there are 8 subnets with 30 hosts each. • When determining how many hosts are needed in each subnet, it is necessary to include the router interface as well as the individual host devices. Each router interface must have an IP address in the same subnet as the host work attached to it.

18. Custom Subnet Masks -The Subnetting

19. Custom Subnet Masks -The Addressing

20. Communicating between subnets

21. Communicating between subnets • When a network is split into two subnets, there are actually two separate networks. • Routers connect networks. In order for a device in one subnet to communicate with a device in the other, a router is required. • The configuration must ensure that interfaces on routers that connect to each other are assigned IP addresses in the same network or subnet, and that clients are assigned default gateways that they can reach.

22. Implementation of IP Addressing in the LAN • The origin, purpose, and function of IPv6

23. IPv6 • CIDR and private IP addressing were developed to provide a temporary solution to the problem of IP address depletion. These methods, though useful, did not create more IP addresses. IPv6 does that. • There were good reasons for IPv6 development. • More address space • Better address space management • Easier TCP/IP administration • Modernized routing capabilities • Improved support for multicasting, security, and mobility

24. IPv6

25. Using Network Address Translation in a Network • The purpose and function of network address translation (NAT) and how it is implemented

26. Network Address Translation (NAT) • Network Address Translation (NAT) allows a large group of private users to access the Internet by sharing a small pool of public IP addresses. • NAT was developed to save registered IP addresses. • NAT also provides security to PCs, servers, and networking devices by withholding their actual IP host addresses from direct Internet access (NAT helps shield users of a private network against access from the outside. ) • The main advantage of NAT is IP address reuse, and the sharing of globally unique IP addresses between many hosts from a single LAN.

27. Network Address Translation (NAT)

28. Using Network Address Translation in a Network • The terms used to describe how packets are transported across a NAT configuration

29. IP NAT Terms • The inside local network refers to any network connected to a router interface that is part of the privately addressed LAN. Hosts on inside networks have their IP addresses translated before they are transmitted to outside destinations. • The outside global network is any network attached to the router that is external to the LAN and that does not recognize the private addresses assigned to hosts on the LAN. • An inside local address is the private IP address configured on a host on an inside network. It is an address that must be translated before it can travel outside the local network addressing structure.

30. IP NAT Terms • An inside global address is the IP address of an inside host as it appears to the outside network. This is the translated IP address. • The outside local address is the destination address of the packet while it is on the local network. Usually this address is the same as the outside global address. • An outside global address is the actual public IP address of an external host. The address is allocated from a globally routable address or network space.

31. Static and Dynamic NAT • One of the advantages of using NAT is that individual hosts are not directly accessible from the public Internet. • What if one or more of the hosts within a network are running services that need to be accessed from Internet connected devices, as well as devices on the local private LAN? • Therefore one way to provide access to a local host from the Internet is to assign that device a Static address translation. • Static translations ensure that an individual host private IP address is always translated to the same registered global IP address.

32. Static and Dynamic NAT • It also ensures that no other local host will be translated to the same registered address. • Dynamic NAT occurs when a router is configured to assign an IP address from an available pool of outside global addresses to an inside private network device. • Dynamic NAT allows hosts assigned with private IP addresses on a network, or intranet, to access a public network, such as the Internet. • Static NAT allows hosts on the public network to access selected hosts on a private network.

33. Port-based Address Translation (PAT) • When an organization has a very small registered IP address pool, or perhaps even just a single IP address, it can still enable multiple users to simultaneously access the public network with a mechanism called NAT overload, or port address translation (PAT). • PAT translates multiple local addresses to a single global IP address. • In PAT, the gateway translates the local source address and port combination in the packet to a single global IP address and a unique port number above 1024. Although each host is translated into the same global IP address, the port number associated with the conversation is unique.

34. IP NAT Issues • The big issue with NAT is the additional work load necessary to support IP address and port translations. • Some applications increase the work load of the router because they embed an IP address as part of the encapsulated data. The router must replace the source IP addresses and port combinations that are contained within the data, as well as the source addresses in the IP header. • With all this activity taking place in a router because of NAT, its implementation in a network requires good network design, careful selection of equipment, accurate configuration and regularly scheduled maintenance.

35. Summary • IP addressing can be tailored to the needs of the network design through the use of custom subnet masks. • Classless subnetting gives classful IP addressing schemes more flexibility through the use of variable length subnet masks. • Network Address Translation (NAT) is a way to shield private addresses from outside users. • Port Address Translation (PAT) translates multiple local addresses to a single global IP address, maximizing the use of both private and public IP addresses.