August 8, 2018

Deliverability 101: What is an IP address?

After our exploration into DNS, IP addresses were mentioned several times and I started thinking, “Maybe I need to explain IP addresses in more detail.” So here we are.

What are IP addresses?

An Internet Protocol (IP) address is a number assigned to any device connected to the internet. When sending email, internet service providers (ISPs) use your individual IP address to identify you and your server. Historically, IP addresses lived in the IPv4 space, formatted in a four-part numbered group, like These are divided into segments (groupings) and assigned to specific businesses of various sizes. These segments can range from an individual IP address (/32) to a very large segment such as a class B (/8) covering more than 16M IP addresses and is comprised of several smaller class ranges made up of 255 IPs each.

As the internet grew in size over the last couple decades, the number of IPv4 addresses started to run out, so a new version of IP addresses was created known as IPv6. While the majority of mail is still sent over IPv4, the transition to IPv6 is underway. IPv6 has a slightly different format than IPv4, consisting of an alphanumeric setup and represents more than 340+ undecillion (yes, that’s a thing, it’s a 39-digit-long number) addresses available.

ESPs and IPs

Most ESPs have been happily mailing over IPv4 for years and will continue to do so for some time. There are, however, a few testing IPv6. They are likely hosting landing pages and microsites right now, but chances are their mail is not exclusively using this IP, as not all ISPs are accepting mail over IPv6 yet.

Congratulations! Now you understand IPs, what they are, and how they are different. We won’t stop there, so now it’s time to discuss shared IPs vs. dedicated IPs. The basic difference between these two ideas is simple because it’s explained in their names. A shared IP is a shared resource used by several of the ESPs customers. These customers can be segregated based on their performance and reputation, a server pool the account was assigned to as they onboarded with the ESP, or a floating system that will move senders as they become more or less trusted as a sender on an ESPs network. This is considered the entry-level space used by most ESPs. Though a shared IP setup tends to weaken email performance for the highest quality senders, it will improve performance for less stellar senders.

But don’t get it twisted. You can’t abuse email without repercussions on a shared IP. ESPs are extremely skilled at sniffing out abusive senders and even predicting unacceptable behavior before it’s conducted, so we suggest you don’t try to get cute.

The second set-up is the dedicated IP, which is reserved for the exclusive use by a single company, or a specific marketing or transactional email channel. Let’s say you send a large volume of email and the success of your business depends on impeccable deliverability: You’ll want to consider a dedicated IP. Because it’s only related to your business, you can define the characteristics of the mail sent over your IP without influence from any other senders. It will paint the most accurate depiction of your mailing practices, and give you complete control over your reputation.

Here’s our bottom line on shared IPs: If you send on a small scale, send irregularly, or simply don’t send email that often, the standard shared IP environment at most top-level ESPs may be beneficial. When mailing via a shared IP, sending with a unique DKIM selector and key will further help differentiate your emails from those senders utilizing the same IPs for sending mail.

A question we sometimes get related to IPs takes this discussion one step further: Should different kinds of emails use different IPs? It’s a good idea to always split dramatically different kinds of email communication onto different IPs. In particular, you’ll want to keep your transactional messaging (receipts, tracking information, password resets, and the like) separate from messages more likely to be caught or flagged as spam, like marketing or sales emails. We get into this more in our Deliverability Guide, so you can read further on how and why you should split emails.

We’ll see you back here next week.

Author: Matthew Vernhout

Matthew Vernhout is a digital messaging industry veteran and Certified International Privacy Professional (CIPP) with more than a decade of experience in email marketing. Matt is 250ok’s Director of Privacy, and he is currently the Vice Chair of the eec, after serving for several years as the Chair of their Advocacy Subcommittee.

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