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Chapter 6 - VLSM and CIDR CCNA Exploration version 4.0 Objectives • • • Compare and contrast classful and classless IP addressing. Review VLSM and explain the benefits of classless IP addressing. Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses Học viện mạng Bách Khoa - Website: www.bkacad.com 2 Introduction • • • Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes IP address space was depleting rapidly – the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR) – CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space. » -VLSM is simply subnetting a subnet Học viện mạng Bách Khoa - Website: www.bkacad.com 3 Classful and Classless IP Addressing • • • Classful IP addressing As of January 2007, there are over 433 million hosts on internet Initiatives to conserve IPv4 address space include: -VLSM & CIDR notation (1993, RFC 1519). -Network Address Translation (1994, RFC 1631). -Private Addressing (1996, RFC 1918). Học viện mạng Bách Khoa - Website: www.bkacad.com 4 Classful and Classless IP Addressing • The High Order Bits These are the leftmost bits in a 32 bit address Học viện mạng Bách Khoa - Website: www.bkacad.com 5 Classful and Classless IP Addressing • Classes of IP addresses are identified by the decimal number of the 1st octet – Class A address begin with a 0 bit Range of class A addresses = 0.0.0.0 to 127.255.255.255 – Class B address begin with a 1 bit and a 0 bit Range of class B addresses = 128.0.0.0 to 191.255.255.255 – Class C addresses begin with two 1 bits & a 0 bit Range of class C addresses = 192.0.0.0 to 223.255.255.255. Học viện mạng Bách Khoa - Website: www.bkacad.com 6 Classful and Classless IP Addressing • The IPv4 Classful Addressing Structure (RFC 790) • An IP address has 2 parts: – The network portion: Found on the left side of an IP address – The host portion: Found on the right side of an IP address Học viện mạng Bách Khoa - Website: www.bkacad.com 7 Classful and Classless IP Addressing Học viện mạng Bách Khoa - Website: www.bkacad.com 8 Classful and Classless IP Addressing • Classful Routing Updates -Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates The reason is that the Subnet mask is directly related to the network address Học viện mạng Bách Khoa - Website: www.bkacad.com 9 Classful and Classless IP Addressing • Classless Inter-domain Routing (CIDR – RFC 1517) – Allows for: • More efficient use of IPv4 address space. • Route summarization. • Requires subnet mask to be included in routing update because address class is meaningless. Học viện mạng Bách Khoa - Website: www.bkacad.com 10