Internet Networking recitation #1 Subnet + CIDR Spring Semester 2008, Dept. of Computer Science, Technion

Administrative Information Course site: webcourse.cs.technion.ac.il/236341 Assistants: Anna Levin Roman Sandler Internet Networking

IP Addressing: Original Classful Scheme IP Address – 32-bit integer globally unique address Dotted Notation: 132.68.37.54 IP Classes – dividing an address to net id and host id The prefix (net id) identifies a network. The suffix (host id) identifies a host on this network. Internet Networking

IP Addressing: Original Classful Scheme Class A – 7 bits to net id, 24 bits to host id 1.0.0.0 – 126.0.0.0 Class B – 14 bits to net id, 16 bits to host id 128.0.0.0 – 191.255.0.0 Class C – 21 bits to net id, 8 bits to host id 192.0.0.0 – 223.255.255.0 Class D – for multicasting Class E – reserved for future use (used for private addresses) Weakness Growth of routing tables in routers Tens of thousands small (class C) networks. Each network must be advertised. Inflexible Lack of a network classes for mid-sized organization (between class B and C). Address space will be eventually exhausted Internet Networking

Subnet Addressing A site has a single IP network address assigned to it, but has two or more physical networks. Different technologies. Limits of technologies. Network congestion. Security consideration. VLAN – separate one physical network into a few logical networks. Administration (e.g. deferent departments in academic institute). From outside it looks like a single network Only local routers know about multiple physical networks inside and how to route traffic among them Host ID is divided into a subnet ID and host ID Accepted as a standard at 1985 (RFC 950). Internet Networking

Subnet Routing When a router gets a packet, it isolates by Net mask the packet net id address. Each routing entry contain a net mask. Routing is done on a longest-match basis. If the packet is destined to other network then the router sends it to another router. Otherwise the router sends the packet to the appropriate host on its attached networks. Internet Networking

Subnetting - Example Network 128.10.1.0/24 H1 H2 Rest of the Internet 128.10.1.1 128.10.1.2 H1 H2 Rest of the Internet R . Network 128.10.2.0/24 All traffic to 128.10.0.0/16 128.10.2.1 128.10.2.2 H3 H4 A site with two physical networks. Using subnetting, R advertise these networks as a single network (thus, R accepts all traffic for net 128.10.0.0) Internal routing is done according to subnet id (i.e. the third octet of the address). Internet Networking

Variable-Length Subnetting Motivation: Consider the case when an organization has a few network of different sizes. When we choose the subnet partitioning, we actually define constant number of possible physical subnetworks with maximum number of hosts on them. Difficult to keep small (waist of subnet numbers) and big (the host id needs more bits) sub networks and there could be unnecessary spending of address space. Solution: Variable-Length Subnetting. A subnet partition is selected on a per-network basis. Internet Networking

Example – Configuring a Network with Variable-Length Subnetting We have a network with IP 202.128.236.0/24 We need to support next sub networks: 6 networks with 26 hosts 3 networks with 10 hosts 4 networks with 2 hosts If we take subnet mask of /27 bits then we can get 8 sub networks of 30 hosts (all 0’s and all 1’s of host addresses are reserved). 11111111.11111111.11111111.11100000 We need only 6 such sub networks. The rest 2 sub networks we will partition by subnet mask of /28 bits. 11111111.11111111.11111111.11110000 We will get 4 sub networks of 14 hosts in each We need only 3 such sub networks. Internet Networking

Example – Configuring a Network with Variable-Length Subnetting The rest we will partition by subnet mask of /30 bits. 11111111.11111111.11111111.11111100 We will get 4 sub networks of 2 hosts in each. Subnet mask #1 = /27 11111111.11111111.11111111.11100000 Subnet mask #2 = /28 11111111.11111111.11111111.11110000 Subnet mask #3 = /30 Internet Networking

Classless Inter-Domain Routing (CIDR) - RFC 1519 Routing destinations are represented by network and mask pairs. Enabling network aggregation; thereby reducing the size of routing table. Examples: Class A networks are followed by a /8 Class C networks are followed by a /24 8 Class C hosts network is followed by /21 Such a network has 21 bits of Net-ID, 11 Bits of Host-ID Contains 2^21 Net IDs, and 2^11-2 = Hosts in Each network. Internet Networking