Chapter 8: Subnetting IP Networks

Subnetting a /24 Network

-/25 row - Borrowing 1 bit from the fourth octet creates 2 subnets supporting 126 hosts each. -/26 row - Borrowing 2 bits creates 4 subnets supporting 62 hosts each. -/27 row - Borrowing 3 bits creates 8 subnets supporting 30 hosts each. -/28 row - Borrowing 4 bits creates 16 subnets supporting 14 hosts each. -/29 row - Borrowing 5 bits creates 32 subnets supporting 6 hosts each. -/30 row - Borrowing 6 bits creates 64 subnets supporting 2 hosts each.

private IPv4 addresses ranges are

-10.0.0.0-10.255.255.255 with a subnet mask of 255.0.0.0 or /8 -172.16.0.0-172.31.255.255 with a subnet mask of 255.240.0.0 or /12 -192.168.0.0-192.168.255.255 with a subnet mask of 255.255.0.0 or /16 note: subnetting selected private IPv4 address space will provide host addresses to cover network needs note2: public addresses used to connect to internet are typically allocated from a service provider

/25 Subnet is applied in a network

-2 LAN segments attached to its GigabitEthernet interfaces -each LAN is assigned 1 of subnets

reason it is big

-8 additional bits -can be borrowed, and, therefore, number of subnets and hosts are simply larger

EX of how to understand how subnetting on the octet boundary can be useful

-Assume an enterprise has chosen the private address 10.0.0.0/8 as its internal network address. -That network address can connect 16,777,214 hosts in one broadcast domain. -The enterprise could further subnet the 10.0.0.0/8 address at the octet boundary of /16 as shown in Figure 1. -This would provide the enterprise the ability to define up to 256 subnets (i.e., 10.0.0.0/16 - 10.255.0.0/16) with each subnet capable of connecting 65,534 hosts. -Notice how the first two octets identify the network portion of the address while the last two octets are for host IP addresses.

within a network, there are different types of devices that require addresses, including

-End user clients -Servers and peripherals -Servers that are accessible from the Internet -Intermediary devices -Gateway

important addresses of first subnet, 192.168.1.0/25

-IPv4 Network address: 192.168.1.0 and contains all 0 bits in host portion of address -First IPv4 host address: 192.168.1.1 and contains all 0 bits + a right-most 1 bit in host portion of address -Last IPv4 host address: 192.168.1.126 and contains all 1 bits + a right-most 0 bit in host portion of address -IPv4 Broadcast address: 192.168.1.127 and contains all 1 bits in host portion of address

situation requiring a larger number of subnets

-IPv4 network is required that has more hosts bits to borrow from

Using VLSM subnets

-LAN and WAM segments can be addressed without unnecessary waste

In an Ethernet, devices use broadcasts to locate

-Other devices -Services

3 primary considerations for planning address allocation

-Prevent Duplication of Addresses -Provide and Control Access -Monitor Security and Performance

Gateway

-Routers and firewall devices have an IP address assigned to each interface which serves as gateway for hosts in that network -typically, router interface uses either lowest or highest address in network

small ISP requires 1000 subnets for its clients

-The network address 10.0.0.0 has a default subnet mask of 255.0.0.0 or /8. -This means there are 8 bits in the network portion and 24 host bits available to borrow toward subnetting. Therefore, the small ISP will subnet the 10.0.0.0/8 network. -As always, in order to create subnets we must borrow bits from the host portion of the IP address of the existing internetwork. -Starting from the left to the right with the first available host bit, we will borrow a single bit at a time until we reach the number of bits necessary to create 1000 subnets. -we need to borrow 10 bits to create 1024 subnets. Specifically, we need to borrow the 8 bits in the second octet and 2 additional bits from the third octet.

VLSM chart

-addressing chart -used to identify which blocks of addresses are available for use and which ones are already assigned -helps to prevent assigning addresses that have already been allocated

End user clients

-allocate addresses dynamically using Dynamic Host Configuration Protocol (DHCP) -reduces burden on network support staff and virtually eliminates entry errors -addresses are only leased for a period of time -Channing subnetting scheme means that DHCP server needs to be reconfigured, and clients must renew their IP addresses -IPv6 clients can obtain address info using DHCPv6 or SLAAC

Variable Length Subnet Masks (VLSM)

-allows a network space to be divided into unequal parts -subnet mask will vary depending on how many bits have been borrowed for a particular subnet, thus the "variable" part of VLSM -using VLSM, always begin by satisfying host requirements of largest subnet -continue subnetting until host requirements of smallest subnet are satisfied

Intermediary devices

-assigned addresses for network management, monitoring and security -must know how to communicate with intermediary devices -they should have predictable, statically assigned addresses

Network are most easily subnetted

-at octet boundary of /8, /16, and /24 -using longer prefix lengths decreases number of hosts per subnet

VLSM subnetting similar to traditional subnetting

-bits are borrowed to create subnets -formulas to calculate number of hosts per subnet and number of subnets created still apply

/24 network address

-commonly subnetted using longer prefix lengths by borrowing bits from fourth octet -provides administrator with additional flexibility when assigning network address to a smaller number of end devices

Preventing Duplication of Addresses

-each host in an internetwork must have a unique address -without proper planning and documentation, an address could be assigned to more than one host, resulting in access issues or both hosts

same subnet mask was applied for all subnets

-each subnet has same number of available host addresses

calculate number of hosts each subnet can support

-examine 3rd and 4th octet -after borrowing 7 bits for subnet, there is one host bit remaining in 3rd octet -8 host bits remaining in 4th octet for a total of 9 bits that were not borrowed

ip address interface configuration

-first IPv4 address for their respective subnets

Using a common addressing Scheme

-first host IPv4 address for each subnet is assigned to LAN interface of router -WAN interface of routers are assigned IP addresses and mask for /30 subnets

calculate number of subnets that can be created form bits borrowed formula

-formula: 2^n -n=bits borrowed -up to 6 bits borrowed -last 2 bits cannot be borrowed from last octet because there would be no host addresses available -longest prefix length possible when subnetting is /30 or 255.255.255.252

calculate number of hosts that can be supported formula

-formula: 2^n-2 -n=number of bits remaining in host field

host on subnet

-host on each subnet must be configured with an IPv4 address and default gateway

Services

-host typically acquires it IPv4 address configuration using Dynamic Host Configuration Protocol (DHCP) -sends broadcast on local network to locate a DHCP server

problem with large broadcast domains

-hosts can generate excessive broadcasts -negatively affect networks -slow traffic operations due to significant amount of traffic it can cause -slow device operations because a device must accept and process each broadcast packet

Network administrators can group devices and services into subnets that are determined by

-location like floors in a building -Organizational unit -Device type -any other division that makes sense for he network

Server that are accessible from the Internet

-many networks, servers must be made available to remote users -most cases, these servers are assigned private addresses internally, and router or firewall at perimeter of network must be configured to translate internal address into a public address

Monitor Security and Performance of hosts

-means network traffic is examined for source IP addresses that are generating or receiving excessive packets -if there is proper planning and documentation of network addressing, problematic network devices should easily be found

large broadcast domain

-network -connects many hosts

two subnet addresses that cannot be assigned to a host

-network address -broadcast address

8.1.5.4: VD- Basic VLSM

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8.1.2.10: Creating 4 Subnets

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8.1.2.11: VD- Creating Four Equal-Sized Subnets

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8.1.2.4: VD- The Subnet Mask

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8.1.2.5: VD- Subnetting with the Magic Number

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8.1.2.8: VD- Creating 2 Equal-Sized Subnets

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8.1.3.6: VD- Subnetting Across Multiple Octets

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8.1.5.7: VD- VLSM example

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difference from VLSM subnetting to traditional subnetting

-not a single pass activity -with VLSM. network is first subnetted, and then subnets are subnetted again -process can be repeated multiple times to create subnets of various sizes

2 considerations when planning subnets

-number of host addresses required for each network -number of individual subnets needed

size of subnet involves planning

-number of hosts -require IPv4 host addresses in each subnet of subdivided private network

each bit borrowed in the fourth octet

-number of subnetworks available is doubled -while reducing number of host address per subnet

Switches propagate broadcasts

-out to all interfaces -except interface on which it was received -receive broadcast, forward it to other switches and other users connected in network

subnetting

-reduce size of network to create smaller broadcast domains -process -prefix length has changed from a /16 to a /24. This is basis of subnetting; using host bits to create additional subnets. -reduces overall network traffic -improves network performance -enables an administrator to implement security policies such as which subnets are allowed or not allowed to communicate together.

VLSM subnetting scheme

-reduces number of addresses per subnet to size appropriate for WANs

subnetting /24 network

-relationship inverse between number of subnets and number of hosts -more bits borrowed to create subnets, fewer host bits available -if more host addresses are needed, more host bits are required, resulting in fewer subnets -number of host addresses required in largest subnet will determine how many bits must be left in host portion -Recall that two of addresses cannot be used, so usable number of addresses can be calculated as 2^n-2.

Planning network subnets

-requires examination of both needs of an organization's network usage, and how subnets will be structured

note for 8.1.2.7

-router interfaces must be assigned an IP address within valid host range for assigned subnet -this is address that hosts on that network will use as their default gateway -very common practice is to use first or last available address in a network range for router interface address

traditional subnetting

-same number of addresses is allocated for each subnet -if all subnets have same requirements for number of hosts, these fixed size address blocks would be efficient -meets needs of largest LAN and divides address space into an adequate number of subnets, results in significant waste of unused addresses -limits future growth by reducing total number of subnets available -not very efficient and wasteful

network addressing scheme to accommodate maximum number of hosts for each network and number of subnets

-should allow for growth in both number of host addresses per subnet and total number of subnets

Servers and peripherals

-should have a predictable static IP address -use a consistent numbering system for these devices

address plan includes determining needs of each subnet in terms of

-size -how many hosts per subnet -how host addresses will be assigned -which hosts will require static IPv4 addresses -which hosts can use DHCP for obtaining their addressing info

subnets/network

-smaller network spaces -most networks are a subnet of some larger address block

Provide and Control Access

-some hosts, like servers, provide resources to internal hosts as well as to external hosts -Layer 3 address assigned to a server can be used to control access to that server -if however, address is randomly assigned and not well documented, controlling access is more difficult

borrowing bits from a /16 address

-start borrowing bits in 3rd octet -going left to right -borrow a single bit at a time until number of bits necessary to create 100 subnets is reached

Performing a network requirement study

-starting point -looking at entire network and determining main sections of network and how they will be segmented

Traditional Subnetting Creates Equal Sized Subnets

-subnets of equal sizes -uses same subnet mask

solution to large broadcast domains

-subnetting

calculate number of hosts example

-there are 7 host bits remaining, so the calculation is 2^7 = 128-2 = 126. -This means that each of the subnets has 126 valid host addresses. -Therefore, borrowing 1 host bit toward the network results in creating 2 subnets, and each subnet can have a total of 126 hosts assigned.

Other devices

-uses Address Resolution Protocol (ARP) -sends Layer 2 broadcasts to known IPv4 address on local network to discover associated MAC address

IPv4 subnets are created

-using one or more of host bits as network bits -done by extending subnet mask to borrow some of the bits from host portion of address to create additional network bits -more host bits that are borrowed, more subnets that can be defined

/24 boundary

-very popular in subnetting -accommodates a reasonable number of hosts and conveniently subnets at octet boundary

Routers do not propagate broadcasts

-when a router receives a broadcast, it does not forward it out other interfaces -R1 receives a broadcast on its Gigabit Ethernet 0/0 interface, it does not forward out another interface -each router interface connects a broadcast domain and broadcasts are only propagated within its specific broadcast domain

certain number of subnets is required

-with less emphasis on number of host addresses per subnet -may be the case if an organization chooses to separate their network traffic based on internal structure or department setup

Classless Subnetting Example

1. Consider the private network address 192.168.1.0/24. The first three octets are displayed in decimal, while the last octet is displayed in binary. The reason for this is because we will be borrowing bits from the last octet to create subnets of the 192.168.1.0/24 network. 2. The subnet mask is 255.255.255.0 as indicated by the /24 prefix length. This identifies the first three octets as the network portion and the remaining 8 bits in the last octet as the host portion. Without subnetting, this network supports a single LAN interface providing 254 host IPv4 addresses. If an additional LAN is needed, the network would need to be subnetted. 3. 1 bit is borrowed from the most significant bit (leftmost bit) in the host portion, thus extending the network portion to 25 bits or /25. This enables the creation of two subnets. 4. two subnets: 192.168.1.0/25 and 192.168.1.128/25. The two subnets are derived from changing the value of the bit borrowed to either 0 or 1. Because the bit borrowed is the 128 bit, the decimal value of the fourth octet for the 2nd subnet is 128. 5. resulting subnet mask for both networks. Notice how it uses a 1 in the borrowed bit position to indicate that this bit is now part of the network portion. 6. dotted decimal representation of the two subnet addresses and their common subnet mask. Because one bit has been borrowed, the subnet mask for each subnet is 255.255.255.128 or /25.

Subnets Based on Organizational Structure

Corporate Network -Sales -Engineers -Technical Support -Human Resources -Executive Management -Research Development

subnetting a /16 network

prefix length-subnet mask-network address- # of subnets- # of hosts -/17-255.255.128.0-11111111.11111111.10000000.0000000-2-32766 -/18-255.255.192.0-11111111.11111111.11000000.00000000-4-16382 -/19-255.255.224.0-11111111.11111111.11100000.00000000-8-8190 -/20-255.255.240.0-11111111.11111111.11110000.00000000-16-4094 (goes up to /30)

subnetting a /24 network table

prefix length-subnet mask-subnet mask in binary- # of subnets- # of hosts -/25-255.255.255.128-11111111.11111111.11111111.10000000-2-126 -/26-255.255.255.192-11111111.11111111.11111111.11000000-4-62 -/27-255.255.255.224-11111111.11111111.11111111.11100000-8-30 -/28-255.255.255.240-11111111.11111111.11111111.11110000-16-14 -/29-255.255.255.248-11111111.11111111.11111111.11111000-32-6 -/30-255.255.255.252-11111111.11111111.11111111.11111100-64-2