IP addresses allocation
Summarize
Summary of IP addresses allocation
The IP addresses allocation feature in ServiceNow enables you to create, view, and update IP Address Blocks and IP Subnetworks, as well as manage allocated IP addresses and active IP address records within your network environment. This capability supports detailed subdivision and management of IP address spaces, crucial for organizing network resources efficiently.
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Key Components
- IP Address Block: A top-level IP range expressed in CIDR notation, representing a managed IP pool.
- IP Subnetwork: A subdivision of an IP Address Block that can be nested to any depth, allowing granular segmentation of address space.
- Allocated IP Address: Individual address slots within a subnetwork’s CIDR, flagged as reserved (for structurally unassignable addresses) or managed (when an IP address record is created from the slot).
- IP Address: The active CMDB record representing a host address or entire subnetwork, created from allocated IPs or using the subnetwork CIDR.
- Managed Network: Defines the network scope in which IP Address Blocks must have unique CIDRs, ensuring no conflicts across different managed networks.
IP Allocation Methods
There are two mutually exclusive methods for creating IP address records within a subnetwork:
- From allocated IPs: Select individual allocated IP slots to create corresponding IP address records for assigning to devices or services.
- At subnet level: Create a single IP address record representing the entire subnetwork, suitable for binding the subnet to a service or interface.
Once one method is used, the other is locked until existing IP address records are deleted. Deleting IP address records preserves the underlying allocated IP slots.
Operational Requirements
Records must be active (Life Cycle Stage: Operational, Status: In Use) before creating subnetworks or allocating IP addresses beneath them. Inactive records prevent these operations.
Reserved Addresses
Addresses that cannot be assigned are automatically flagged as reserved based on protocol-specific rules:
- IPv4 reserves the network and broadcast addresses, except in /31 and /32 subnets where no reservations occur.
- IPv6 reserves the Subnet Router Anycast address; no broadcast addresses exist, and /128 subnets have no reserved addresses.
CMDB Relationships
IP Address records maintain relationships in the CMDB that reflect their origin and hierarchy:
- When created from allocated IPs, records link to both their allocated slot and subnetwork.
- When created at the subnet level, records link only to the subnetwork.
These relationships are accessible in the Related Items panel and Dependency View, aiding in network resource tracking and management.
Practical Benefits for ServiceNow Customers
- Streamlines IP address space management with clear hierarchy and subdivision.
- Ensures unique and conflict-free IP allocation within managed network scopes.
- Supports flexible IP address assignment methods tailored to operational needs.
- Automatically manages reserved addresses to prevent assignment errors.
- Integrates IP address data into the CMDB, enhancing visibility and relationship tracking for configuration management.
Using the IP address allocation, you can create, view, and update IP Address Blocks and IP Subnetworks. You can also manage allocated IP addresses and IP addresses.
IP address tables
- IP Address Block
- A top-level range of IP addresses expressed as a CIDR. The user interface label IP Address Block refers to the underlying Managed IP Pool class.
- IP Subnetwork
- An operational subdivision of an IP Address Block. An IP Subnetwork can itself contain further nested IP Subnetworks, which lets you subdivide IP address space to any depth.
- Allocated IP address
- A single address slot within an IP Subnetwork's CIDR. Allocated IP address records carry two flags. Is Reserved is set automatically for structurally unassignable addresses. Is Managed is set automatically when an IP Address record is created from this slot.
- IP address
- The active CMDB record for an address. It can represent a single host address within a subnetwork, created from an allocated IP. Alternatively, it can represent the entire subnetwork by using the subnetwork's CIDR as its value.
- Managed Network
- A network scope within which an IP Address Block's CIDR must be unique. The same CIDR may exist in different Managed Networks without conflict. The Managed Network assigned to an IP Address Block propagates to its IP Subnetworks and cannot be changed at a lower level. For more information, see Create a managed network.
IP allocation methods
An IP Subnetwork uses one of two methods to create IP address records. The two methods are mutually exclusive: A subnetwork may use one or the other, but never both at the same time.
- From allocated IPs: The user selects one or more allocated IP records on a subnetwork and creates a corresponding IP address record for each selected slot. Each new IP address record uses the host address as its value. This method is appropriate when individual addresses are being assigned to devices, interfaces, or services.
- At subnet level: A single IP address record is created for the subnetwork as a whole. The IP address record uses the subnetwork's CIDR as its value, rather than a single host address. This method is appropriate when the subnetwork is bound to a service, port, customer, or interface.
After the first IP address record is created on a subnetwork by either method, the other method is locked for that subnetwork. To switch methods, delete the existing IP address records. Deleting IP address records preserves the underlying allocated IP slots.
Active state required for operations
A record must be in an active state before child records or IP allocations can be created against it. A record is active when the Life Cycle Stage is Operational and the Life Cycle Stage Status is In Use. Any other combination of values prevents the creation of subnetworks beneath the record or the allocation of addresses from it.
Reserved addresses
When allocated IP records are generated for a subnetwork, addresses that are structurally unassignable are flagged automatically with Is Reserved = True. The rule differs by protocol. For IPv4, the first address (the network address) and the last address (the broadcast address) are reserved. A /31 IPv4 subnet reserves no addresses (RFC 3021); a /32 IPv4 subnet reserves no addresses. For IPv6, only the first address — the Subnet Router Anycast address (RFC 4291) — is reserved. IPv6 has no broadcast address. A /128 IPv6 subnet reserves no addresses.
CMDB relationships
Every IP Address record participates in standard ServiceNow CMDB relationships. The relationships created depend on the method used. When IP Address records are created from selected Allocated IPs, the system writes two relationships per record. The first is an IP Address Manages Allocated IP relationship, linking the new record to its source slot. The second is an IP Subnetwork Contains IP Address relationship, linking it to the subnetwork. When a single IP Address record is created at the subnetwork level, only the IP Subnetwork Contains IP Address relationship is created.
Both relationship types are visible on each IP Address record's Related Items panel and in the Dependency View.