In this course we consider IP protocol (RFC791) which is a datagram service. It is the most commonly used network layer protocol on the Internet.

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1 The Network Layer The network layer is responsible for moving bits around the network. Fundamentally there are two types of network layer architectures datagram based and virtual circuitbased(vc). The first is connectionless and stateless, while the second is connection based and preserves state information. By way of analogy, the datagram services are like postal services (mail envelope and hope for the best), while VC based services are like telephone services (establish a connection, talk, and then bring down the connection). In this course we consider IP protocol (RFC791) which is a datagram service. It is the most commonly used network layer protocol on the Internet. The functions performed by the IP protocol roughly fall into three categories. IP Layer functions Routing ICMP Error handling messaging IP Protocol Addressing schemes Datagram formats Packet handling conventions Routing is a vast topic. We will not go into details of routing at this point. Basically there are two types of routing architecture in existence, intrta domain routing and inter domain routing. An administratively self contained domain (such as newpaltz.edu) is usually called an Autonomous System (AS). Thus there is intra AS and inter AS routing. Intra AS routing is relatively easy, and most use either RIP (Routing Information Protocol) or its upgrade, the OSPF (Open Smallest Path First) protocol. The standard routing protocol for inter AS routing is BGP (Border Gateway Protocol Version 4).

2 Addressing of hosts in the IP protocol is closely and subtly linked to routing. IP Addresses, mask bits and subnets IP Version 4 addresses are 32 bits long. For our convenience its is normally represented as four byte segments in decimal form, separated by dots (dotted decimal notation). For example, the IP address is usually written as When trying to understand IP numbers, focus on the bits or binary representation, not on the decimal notation humans use. A lot of confusion can be avoided if you remember that classification and use of IP addresses depends on the bits, not their decimal representations. Remember that the byte boundaries defined in the above dotted decimal notation is just for human convenience. We could have decided to use a 2 byteboundary based decimal notation, in which the above IP will look like It just is more convenient for us to use a 1 byte boundary based decimal notation to write and remember IP addresses. IP addresses have two parts, a network part and a host part. The network part identifies the network that the host is a part of. The host part identifies the particular host within the network. This is much like phone number area codes identifying the geographical region where the phone is located, and the rest of the number uniquely identifying the particular

3 phone. Within a region identified by an are code, there are further subdivisions in the phone number scheme; similarly for IP numbers there are subnets within networks New Paltz Ulster/Dutchess area The IP addressing scheme was originally based on classes there were class A, class B, class C and multicast IP address classes. These classes have been superseded by Classless Inter Domain Routing (CIDR), which allows greater flexibility. In CIDR (as in previous class full schemes), a network is specified by the number of netmask bits. All interfaces in a given network will have the same netmask bits. For example, the newpaltz.edu domain has a 16 bit netmask, so that every interface in the newpaltz.edu network will have their first 16 bits identical. We identify the newpaltz.edu domain by the CIDR notation /16. This means that the first 16 bits of all IP addresses in this domain will be , or The second half of is the host part. Hosts can get any address in this part, except for the lowest ( ) and the highest ( ); the lowest is reserved to identify the network, called network address, and the highest is called the broadcast address, which is reserved for broadcasting throughout that network. The full description of a host then becomes, for example /16, meaning that the host resides in a network, every host of which will have ts first 16 bits Another notation for netmasks is the following: to specify a 16 bit mask, you use that is, Thus the CIDR notation /16 is equivalent to saying with netmask Sometimes the hexadecimal notation with netmask ff.ff.0.0 is also used. Remember that in the /16 domain, there are 2^16 2=65534 possible host addresses that can be assigned (2 was subtracted from 2^16 for network and broadcast

4 addresses). To make administration simple, this domain can be further subdivided by specifying netmasks. So for example, the /16 domain can be further subdivided by specifying that a subnet in this domain will have a 20 digit netmask. It is up to the administrator of the big domain to allocate the 4 extra netmask bits beyond the 16 bits that's common to all hosts. For example, suppose that a new sub domain, called CompSci.newpaltz.edu, is alloted with the network address /20 Thus the first 20 bits of all hosts in this new subdomain will look like: 20 mask bits all hosts in this subnet will have these 20 bits the same. these bits can be allotted to hosts The network address of the domain we just created is /20. To use a different terminology, the subnet has network address with netmask ( ). What is its broadcast address? To get the broadcast address we set all the host bits to their highest values, 1. This gives us All IP addresses in this subnet will fall between and For example, and are possible IP addresses. Since there are 12 bits available for hosts, you can have 2^12 2=4094 host IP addresses in this subnet Network address Broadcast address Host IP addresses

5 Note that in the above example, the administrator of the /16 domain assigned the next 4 mask bits, 1001, to create a sub domain. In fact, these four bits could in principle be assigned in 2^4=16 different ways. This will create 16 different subdomains. The 16 possibilities ( for the last of the four mask bits) are 0000, 0001, 0010, 0100, 1000,..., As an example, the sub domain with network address ( /20), has broadcast address ( ) and all possible host IPs fall between these two extremes. Exercise: Suppose you have a network /20. If this network is to be sub netted with a 23 bit mask, (a) how many subdomains (subnets) can be created? (b) What are the ranges of IPs that can be assigned within each subnet? Solution: Original mask bits (20) New mask bits (23) xxx Remember that the first 20 bits are fixed, given to you. In sub netting, you have flexibility to allocate only the next three bits. These three bits, denoted by xxx above, can be assigned in 8 different ways, 000, 001, 010, 100, 011, 101, 110, 111. Each of these gives rise to a new sub domain.

6 Network address of new subnets Broadcast addresses of new subnets The broadcast addresses for each subdomain can be found by setting the host bits (in black) to 1. The host IPs in each subnet is a value between the network and broadcast address of the subnet.