Subnet computing guide: Planning network using IPv4, CIDR and host range

What exactly does CIDR describe?

The IPv4 address is 32 bits./24 Show first 24 places as network and left 8 places as host places, corresponding to the mask 255.255.255.0。 The host is all 0, the network address, all 1 is a broadcast address, and the traditional IPv4 subnet usually has the number of addresses available 2^Host Location - 2。

The “nable host number” is a traditional concept and does not mean that these addresses can be distributed safely. The cloud platform may retain several addresses, and network equipment may also give the first address to the gateway, and the last address for broadcast or reserved purposes. Planning should be based on the Platform documentation and actual equipment behaviour.

Border between Network and Host Address

The given IP and prefix, the network address is the same as the IP and mask by bit. Here. 192.168.10.77/26 For example, (26 addresses per paragraph 64, with network boundaries 0, 64, 128, 192; 77 landed 64 to 127, so the address is 192.168.10.64, the address is 192.168.10.127, the traditional range is 192.168.10.65 Present. 192.168.10.126。

Prefixes are not a direct expression of " number of hosts available " . To accommodate 50 devices, there are 62 traditional available addresses; 27, 30, not enough; 25/This is enough, but it will expand the broadcasting domain. The address planning should take into account both growth, isolation and route aggregation.

What do you have to do before you use the tools?

Align the demand in three rows: IP, a clear CIDR prefix or mask, the purpose of this address. For example, “storage cameras, about 40, gateway pre-set addresses, will grow to 55 in the future”. Don't fill it out with your feelings when you don't have a prefix. /24； The prefix is extrapolated from the host number and the quarantine target before the calculator is validated.

There are three different addresses:

• Private Address：10.0.0.0/8、172.16.0.0/12、192.168.0.0/16, suitable for an internal network, which usually requires NAT.
• Internet address(b) Distribution by operator or cloud manufacturer, by route and security, and not by free transfer to other tenants.
• Special AddressRecycle, local link, grouping etc. are used for specific purposes and cannot be assigned to a normal host address.

The kg network calculator operation step

1. Open IP subnet calculator, confirm the input box and the CIDR prefix drop-down box.
2. Fill in an IPv4 address in the IP address input box, e. g. 10.24.13.37。 Just enter the address, don't put it /21 paste together unless the page explicitly supports this format.
3. Select in Prefix Length (CIDR) 21, or select the corresponding value according to the actual mask. Don't just look at the last part of the address to guess the subnet.
4. Click " Count Now " to read the results of the web address, subnet mask, broadcast address, range available and number of hosts.
5. (b) Review using boundary values: input the network address itself and the broadcast address, the result of which should remain in the same section; Move IP to the next boundary and the network address should change.
6. Click to copy the results and post the complete results to the change order; The input IP, prefix, calculation time and planned use are also recorded.
7. To check conflicts, the existing and candidate segments are counted separately, comparing their start-up range rather than only the string prefix.

Page default processing IPv4. It does not simulate the maximum prefix matching in route-based tables, nor does it automatically split multiple equal length or transformation of long-wire networks; These tasks need to be combined with router configuration and address planning forms.

Input output contrast: read the entire network from one address

Input：10.24.13.37prefix /21。

/21 The mask is... 255.255.248.0The third eight-team is eight. 13 landed between 8 and 15, so the output should read:

Chile10.24.8.0
Radio address:10.24.15.255
AvailableScope：10.24.8.1 - 10.24.15.254
Total number of addresses：2048
传统Number of hosts available：2046

The back-to-back check is important: if you're wrong, /24The network will become 10.24.13.0The radio became... 10.24.13.255, the only available addresses are 254. Both results are like "reasonable IP" but the size of the network and the effect of isolation are completely different.

Subnet planning isn't just a number.

Estimates based on the number of hosts. When 100 terminals are required, 62 addresses are insufficient for the 26/26 and 126 addresses for the 25; If the number of units is expected to increase to 180 a year, it should be assessed directly by 24 or more VLANs, rather than by a temporary build-up resulting in a renumbering.

By broadcasting domain control. The more equipment is exchanged in the network, the greater the broadcast and malfunctioning impact. Office terminals, cameras, visitors Wi-Fi, servers should normally be split and not all should be placed in a /20, even if the total number of addresses is sufficient.

Clock by route. Continuous /24 It can be condensed into /22(b) Simplify upstream routes; But aggregates must remain continuous and security strategies must express the range of aggregations.

Keep the rules by cloud platform. Cloud manufacturers often keep network addresses, broadcast addresses and several platform addresses in each subnet, and the actual availability will be compared 2^n - 2 Less. The traditional number of calculators can only be used as a base, ultimately based on cloud files and control tables.

Theme Unique Error Table

When “frequently accessible, occasionally inaccessible”, priority is given to checking whether the mask is consistent. The two hosts IP are correct but not the same mask, and one may think the target is local, while the other sends the traffic to the gateway.

Real scene: four checks before change

VLAN new. The network team gave it to me. 172.20.40.0/23The business is expected to accommodate 300 units. 23 Only 510 traditional available addresses, sufficient; But make sure the switch, the DHCP and the firewalls are covered in the same code. 255.255.254.0and leave a gateway and infrastructure address.

Point to point link. The two routers are used between 30, the network and broadcast addresses are not distributed, and one address is taken from each end. Waste can be reduced if equipment is supported / 31, but change orders must include equipment and agreement support.

Container clusters. Node subnets, Pod and Service do not overlap. The CIDR candidate starts with a separate range, then compares the company VPN, the office network and the cloud VPC.

Visitors network isolated. Visitors may only need 24/24, but firewall rules are required to block the internal RFC 1918 segment. Subnet calculators can give range, but do not automatically generate ACLs; Security strategies still need to be manually reviewed.

Fault check. One terminal. 192.168.50.200/25The gateway is here. 192.168.50.1/24。 The separate calculations show that the local range is considered to be 192.168.50.254, and the gateway configuration may consider it to be the other side, repairing the DHCP mask and then clearing the lease.

Security and privacy borders

• IP addresses may themselves be internal asset information. The tool is calculated locally in the browser, but not publicly shared in the production segment and in the amplification with the host name.
• The calculator does not verify whether you own the section or check the actual configuration of the router, DHCP, firewall; The Internet users must check the site before going online.
• The public network address range is not used as a “whatever you want”. Authorization is required for publication, scanning and route announcement.
• (a) The result is to remove unnecessary host names, cloud accounts and supporting documents when copies are made; It is sufficient to retain prefixes, scope and change of purpose.
• This paper only discusses the IPv4 base division and does not replace the IPv6, NAT, VRF, BGP or the cloud manufacturer's exclusive address retention rule.

How to choose relevant tools

(b) The subnet calculator if the mission is “the section of the host”; (a) A range of clusters or scripts if the mission is “conflicting over multiple segments”; If the task is “how to forward route”, check the route chart and the maximum prefix.

FAQ

Pre-delivery inspection

Each segment should be written as “Use, CIDR, web address, broadcast address, range, gateway, preservation address, responsible person”. The mathematical check is done using a calculator, which allows network and security managers to review overlaps, route and access controls before eventually changing DHCP or firewalls.